As I've written before, I don't think Harry's scar is a horcrux. Basis? I think horcrux creation involves some kind of spell, and when Voldemort zapped Harry and his family, he didn't perform any spell; however, I don't remember where I read that or if I read it at all. I'll have to re-read the portions of Book 6 that discuss horcruxes.
Travis over at Sword of Gryffindor has an interesting theory. In his quest for horcrux clues, he references a passage in Book 1, Harry Potter and the Sorcerer's Stone.
Travis takes us all the way back to Harry's first night at Hogwarts. He'd just finished eating a generous meal after having been sorted into Gryffindor House. Just after the sorting, Harry makes eye contact with the Potions Master, Professor Severus Snape, whose ominous presence even this early in the game portends shadowy, creepy, and mysterious things. When Harry's scar begins to hurt, he thinks it has to do with Snape. Professor Quirrell, sitting beside Snape, is the reason. More precisely, Lord Voldemort is the reason.
Let's look at the passage:
Perhaps Harry had eaten a bit too much, because he had a very strange dream. He was wearing Professor Quirrell’s turban, which kept talking to him, telling him he must transfer to Slytherin at once, because it was his destiny. Harry told the turban he didn’t want to be in Slytherin; it got heavier; he tried to pull it off but it tightened painfully - and there was Malfoy, laughing at him as he struggled with it - then Malfoy turned into the hook-nosed teacher, Snape, whose laugh became high and cold - there was a burst of green light and Harry woke, sweating and shaking. (Harry Potter and the Sorcerer’s Stone, Scholastic, 130)
At this point, no one knows that Voldemort is inside the turban. Travis surmises that this dream is very significant. He believes it may be a clue that the scar on Harry's forehead is a horcrux.
We know that Voldemort has possessed Quirrell and is hiding inside his smelly turban, waiting for a chance to steal the sorcerer's stone so he can gain immortality. Why would Harry just happen to dream the turban is talking to him, when near the end of the book, the turban actually does "talk" to him? Coincidence? No such thing.
We know there's a connection between Harry and Voldemort, so I believe Harry was reading V's thoughts the way he's been able to throughout the series.
Travis and I agree on the connection between the two, but here is where we diverge. I don't believe the connection necessarily means the scar is a horcrux. The dream was foreshadowing Voldemort's plans as well as the fact that he used the Avada Kedavra curse on Harry and his parents. We learned for the first time in Book 4, Harry Potter and the Goblet of Fire (correct me if I'm wrong), that Voldemort used this killing curse, which explains why Harry had faint memories of seeing green light as a baby and why he sees the same light in this dream.
In Book 5, Harry Potter and the Order of the Phoenix, we learn that Voldemort is an accomplished Legilimens (mind reader) and that Harry "inherited" this trait from him. He was able to read Voldemort's thoughts then, and he's doing the same in this dream.
Travis says the dream is a clue that the scar is a horcrux, and the horcrux is the reason the two have a connection. I say the dream is only foreshadowing the ending of Book 1 and Harry's ability to practice legilimency, as revealed in a future book.
That was fun. 
January 30th, 2006 at 11:28 pm
Update: LaShawn Barber has bounced off this post to discuss the dream as well, and I appreciate her comments, so go read them! I tried to comment on her site, but I'm getting an error message, so here's a quick clarification: I'm not entirely convinced that this is evidence for Harry's scar being a horcrux. I was initially quite opposed to the idea. Discussions with John Granger have pushed me in the other direction, however, and I thought I'd work the idea out and see where it gets me. But let's just say that if the Harry's-scar-as-horcrux theory were the dropoff of a great big cliff, I'd be hanging onto the edge by one hand.
January 31st, 2006 at 10:31 am
That's a good theory you know. The one that Harry's scar might be a horcrux. 'One cannot survive while the other lives." That's what the prophecy says, so it's possible that maybe Harry has to die so that Voldemort will too. But I guess that's sort of a morbid thought.
January 31st, 2006 at 11:19 am
I think it's too complicated. The beauty of Rowling's writing is that everything has a simple explanation, and the scar-as-horcrux theory severly complicates matters. Her characters also remain true to their natures.
Voldemort is a Jackdaw - he collects rare and significant objects for his horcruxes, and keeps them well-hidden. Harry is rare and significant, but not in the superficial ways that would matter to Voledmort.
Plus, Voldemort had his horcruxes in place before he attacked the Potters; else he wouldn't have survived.
Although, since something of Voldemort passed into Harry when he was attacked, perhaps Harry became an unintentional horcrux. Hmm…
January 31st, 2006 at 11:31 am
Thanks for commenting, everyone.
Jared - Perhaps V didn't have all the horcruxes in place. Some theorize that he planned to create one the night he killed Harry's parents, but Lily's "ancient magic" put a stop to that.
January 31st, 2006 at 1:26 pm
Jared, there are very few people who theorize that Harry's scar is an intentional horcrux.
And actually, the theory of Harry's scar being a horcrux UN-complicates things, in my opinion. It actually explains a whole lot of unexplained things in one fell swoop, so to speak (the strange scar, the mental connection with Voldemort [and Nagini, if she is a horcrux], parseltongue, the Sorting Hat's initial thought of putting him in Slytherin, etc.). It's almost a perfect "Occam's Razor" explanation for a lot of unexplained things.
January 31st, 2006 at 11:37 pm
I am wondering if, when Tom Riddle goes to visit Dumbledore to 'apply' for a teaching post he was secretly able to turn Griffindor's sword into a horocrux…Read that passage in Half Blood Prince again and ponder.
February 1st, 2006 at 4:03 am
I'm not going to worry too much over the origin of Harry's scar. I enjoy the books, movies and the video games about Harry Potter well enough without giving them any excess thought. Actually when I read, watch, or play any of the above I excercise an absolute minimum of thought and just enjoy.
February 1st, 2006 at 8:22 am
Jim, that's the joy of being a fan of the series. Some people theorize it to death; others don't. As you know, there's a whole legion of fans out there who go into in-depth analysis of Harry Potter. What I do on this blog is just kid's stuff.
For instance, Lumos 2006 is a Harry Potter symposium attended by adults. They present academic papers, talk about Harry Potter from a legal/cultural/social point of view, etc.
I'm hopelessly caught up in theorizing. And I'm loving every minute of it.
February 1st, 2006 at 1:34 pm
I've heard the "Harry (or his scar) is a horcrux" theory, and I think there is a very easy explanation why he isn't, and in fact, couldn't be. Voldemort's immortality depends on the "survival" of his horcruxes. If Harry, or his scar, were in fact a horcrux Voldemort would not be trying to kill him.
February 3rd, 2006 at 12:08 pm
Lisa, there is an equally easy to explain response to that objection. In Goblet of Fire, Voldemort did not yet know Harry was a horcrux. Hence, he tried to kill him. By the end of Order of the Phoenix, LV had disocovered this fact, and by Half-Blood Prince, LV has actually STOPPED trying to kill Harry.
In fact, that's a huge question, in my opinion. Who's easier to kill: Dumbledore or Harry? Surely Harry. Why would LV then commission Draco to kill Dumbledore? Especially when Harry is the prophesied vanquisher of Voldemort?
February 4th, 2006 at 6:35 pm
You don't think that LV was powerful enough to know whether or not he was missing part of his soul? I would think he'd know from the moment the spell backfired. And, every time LV tried to kill Harry, he failed. So, wouldn't it make sense to take another tack? Harry's mentor and protector was Dumbledore, the only wizard LV ever feared. It would make sense to kill Dumbledore first, so that Harry is left unprotected. Plus, Dumbledore's been about the business of locating horcruxes to destroy them, and is passing on crucial information and preparation to Harry. In LV's mind, with Dumbledore out of the way, his path back to power is clear, except for one Harry Potter… who should be easy pickin's in LV's mind because the young man is obviously no match for him (alone).