Actually, I have to disagree with Julie on this one (sorry...) I think it is more appropriate to the Other Pairing category with an AU warning. The romance is the main part. Readers aren't probably going to be trolling the AU cat looking for a Snape romance. That's just me, though. If the other mods say AU category, go with that.

EDIT: I say this being the Snape/Hermione mod. If all those stories had to go to the AU cat now, the category would cease to exist.