If you haven't read The Half Blood Prince yet, then first, we're super sorry about those flagrant spoilers above, and second, you probably don't know about Horcruxes yet. Horcruxes are magical artifacts that supposedly contain fractions of Voldemort's soul. It's expected that the main narrative of The Deathly Hallows will be about Harry's hunt for the remaining Horcruxes before his final confrontation with Voldemort. If this is true, it means that the final Harry Potter Book will have the same plot as basically every RPG ever made, what with Harry having to "collect the Seven Important Things" to defeat the "loosely explained magic barrier" which will allow him to access the "final dungeon," and in the end he'll still be a "massive nerd."
The identity of all the Horcruxes isn't entirely known yet. Most are suspected to be mundane personal objects (lockets, diaries, body-grooming razors, etc.). However, one of the most popular theories floating around the Internet right now is that Harry himself is a Horcrux.
This fits somewhat with what's known about how Horcruxes are made. A Horcrux has to be made after a murder, it can be a living creature, and it's usually something that's meaningful to its creator in some way. All of these rules could be said to apply to Harry (or his scar). Further supporting this, Harry's scar does have a weird spooky connection with Voldemort, and Harry himself has said that Voldemort put a "piece of him inside me," before quickly adding "but not in a gay way."
But-and much like the one on Aunt Marge, this is a big but-since he was resurrected, Voldemort himself has tried to kill Harry at least twice. He wouldn't do that if Harry contained an important and irretrievable part of his soul. That's a little bit like banging your penis on a hot stovetop to get rid of crabs.
The only way this theory works is if Voldemort didn't know he'd made Harry his Horcrux... which would imply that Voldemort is pretty casual with pieces of his soul. The guy probably listens to Fall Out Boy by the ton.
Our "100% Lock, Money-in-the-Bank" Prediction: Harry's not a Horcrux. For Voldemort to have accidentally created a Horcrux after what was, for him, just a routine double-murder, implies that murderers could have been accidentally making Horcruxes (and thus becoming immortal) for centuries. That's absurd- not Quidditch-level absurd, but still pretty out there.