It seems like half of the thread asking whether or not Anakin fulfilled the prophecy brings into question the nature of the Force. So lets get this out there.
There are two options that I'm aware of: (a) The Dark Side in itself unbalances the Force due to it being "unnatural," and so balance is achieved when all Sith are dead, or (b) The Jedi didn't truely understand the nature of the Force in that, in reality, balance is achieved when The Light and Dark are equal.
I, personally, believe option (b). The fogey old Jedi thought they were so good, but there is no good without bad. There is no day without night.
Don't foget to vote in the poll.