Anything is possible, but Bioware has been known to pull twists on the player before. For example, in ME1 Saren was heavily hyped as the main villain even though it eventually turned out he was a pawn for the Reapers. The only thing for certain about the main villain for ME2 is that we'll have to wait until we play and finish the game to know.
As far as I know, Grayson was killed by Anderson. Though it is possible the Reapers might re-animate his body to do their bidding.
Grayson is very dead.
^So was Shepard and you all saw what happened
He doesn't become a reaper. He becomes something like the post portem Saren. But is strangely killed by a shotgun. Then the Alliance took the body to conduct tests on it and if it came back to life I think they would say something about in the epilogue. Which they didn't. :)