6 total is what Dumbledore guessed.
We know the diary was one, and the locket is another.
One guess as to what they are
SPOILER BELOW:

Marvolo’s ring - Destroyed
Riddle's diary - Destroyed
Slytherin’s locket
Hufflepuff’s cup
Gryffindor artifact
Ravenclaw artifact
The bottom two are really the ones that are uncertain.

Oh also I'd say Harry is not a horcrux because Voldemort's been spending so much time trying to kill him. Why try to destroy the one thing that would help keep you alive? Also it was Harry that caused Voldemort to lose his powers (again while Voldemort was trying to kill him). I'm not sure there's any reason for Voldemort to target Harry so unceasingly if he's a horcrux. V. thinks that Harry will destroy him (and has since Harry's infancy) why try to make him into a horcrux?

Random question: I see everywhere that it's widely accepted that the Grey Lady is Ravenclaw's ghost but she's not mentioned once in any of the books, either the UK or the US version. I know because I've combed them through carefully. So where the hell is this coming from?

http://en.wikipedia.org/wiki/Hogwarts_ghosts

Also found this http://www.hp-lexicon.org/wizards/ghosts.html
It also says that Ron and Harry meet her in the Chapter 12: The Mirror of Erised

thats good..i feel that way too..maybe he's like his uncle or something..
hermoine and ron are going to hook up whenever they get pass the awkward teen stage...someone else is probably going to die as much as we dont want to admit..

But it's never said explicitly that she's the Grey Lady, Rowling had to explain it later. Total crap, she needs to put her in the books properly. I mean come on, if Hufflepuff has the Fat Friar and Slytherin the Bloody Baron and Gryffindor has Nick, it would not be too much effort to actually name the Ravenclaw house ghost when she's referenced.
Either way, thanks for clearing that up for me. It's been bugging me for ages.

Hey Drollefille, if you want to hide the text so it's invisible, this is the color that you use for the font:
color=#F5F5F5 (I got it from a hexcode site).
:)

Allow me to run with the "Scar is a horcrux" theory for lil' while longer....: :)
When Voldemort was killing the Potter parents, I think Voldemort might have unintentionally wounded Harry, and that scar ended up being a horcrux, since Voldemort accidentally transferred over some of his other powers to Harry, such as the ability to speak Parseltongue (snake language), Harry's scar throbs or "flares up" when V. is near, (and then also, in Book #1 in Ollivander's Wand Shop, the wand that "chose Harry" was the brother-wand of Voldemort's wand). If Voldemort were to kill Harry, would the (alleged) scar-horcrux / piece of soul go back into Voldemort and reunite with the rest of his fractured soul, or would that horcrux piece of soul just die too? Hmmm.....

If the soul went back to the original owner, they wouldn't spend so much time trying to destroy the horcruxes would they? I mean, they have said-once the horcruxes are gone, Voldemort just needs to be killed off to get rid of the last piece of his soul.

Ah...you are right...I didn't think of that, which would make Harry's head kinda like the diary....one chop to the head and that would be it. Ok...yeah, I am convinced now that the scar is "special", but is not a horcrux.

True. It was in her notes and hasn't made it into the books.
Thanks, we'll see if I remember that :D
It makes sense. And, just watched Goblet of Fire, so someone tell me if this isnt' in the books officially. Harry survived Avada Kedavra right? Not something you use if you're trying to make a horcrux.

Yes, Harry survived the killing curse. It's also noted that Voldemort wouldn't just pick any object for a horcrux, it would have to be special. I believe the murders he committed would have been the same-monumental and important. So then we could theorize that Harry's almost-murder might have been to create a horcrux, even though it would not have been Harry himself. I mean, Voldemort heard part of the prophecy so what better way to achieve immortality than creating a horcrux from the murder of the one you felt might stop you in your path?

So, going on the theory that a horcrux was created when Harry & Voldemort first "fought" (and it's not the scar)...I'm thinking that the "last" horcrux is something that Harry's had with him since that night, and Voldemort's battles with Harry so far is about getting the horcrux back into his control.

If this is true, then what item has Harry had with him since he was a baby and could possibly be the horcrux?

ooooooooo! Good theory, sister! Something of his mum's?
I haven't read the books that indepthly, so I wouldn't know.

I agree it's not the scar. Many people get curse scars, that's not just in Harry's case (Ron said so, I forget which book). But Harry hasn't had anything that was his since he was a baby, except the blanket he was wrapped in (long gone) and the note from Dumbledore (which was Petunia's). There's his dad's invisibility cloak, but that's been with Dumbledore until it got to Harry. So I don't know about this one. I still think that Voldy never got to make the last horcrux b/c he never succeeded in killing Harry. So maybe there'll turn out to be 5.

We all know he has his mother's EYES! :D