i agree with a theory that half-blood prince is Draco Malfoy. Because in book four Narcissa Malfoy-Draco's mother, didnt really look happy at the Quiddich Cup, therefore i think, she doesn't love Lucius (who would?). and if that's the case, she might've sleep with someone else (muggle?), just to get her thoughts of her unhappy marriage, and get pregnant,and after that say that it is Lucius's son, which it wouldnt be, but it would make Draco half-blood, even though he wouldn't know it.
Also there's that "prince" thing, and who else could more "princy" (is that even a word?)in Hogwarts, more then Draco?
also i would like to say something to the theory that Half-Blood prince is Gilderoy Lockhart. In fact i think, it is possible too, but i kind of always liked Malfoy better (love blonds!) :)