Gamespot has more info on the movie.
FFVII: Advent Children Info
According to them, the short haired guy is not Sephiroth but instead is a brand new character. I don't know... Maybe a re-incarnation?
It also says that this takes place two years after FFVII in a more or less peaceful world except for a physcial illness called Seikon-Shoukougun. Cloud is living alone apparently, so no word if Tifa and co. will show up or how big a role they will play...
*shrugs and walks away*