I've just re-read the Goblet of Fire and had an idea that's inspired me to register!
OK - the follwoing two points have been raised
1. Is Harry a Horcrux?
2. How come Voldemort didn't use a Horcrux to ressurect himself at the end of GoF?
What if... Harry was a Horcrux, and by using his blood in the flesh, blood and bone chapter, he's been 'used up' and now isn't one any more?
It occured to me because near the end when Harry is telling Dumbledore and Sirius what happened in the graveyard, he tells them that Worntail took his blood
'for a fleeting instant, Harry thought he saw a gleam of something like triumph in Dumbledore's eyes'
Triumph because he's already considering the Horcrux theory and has just realised that Voldemort has used one up??
I've thought about it and the only part that doesn't stand up as a theory is that Dumbledore didn't mention it to Harry, and made him think that there are still 4 Horcruxes to find....
Thoughts??