One theory I've heard of relates to the Chaos Theory -- assuming that the universe in general has an increasingly chaotic 'state', if you can align your own 'state' to a previous, more ordered quantum state, you go 'back' in time.
Either that or you're going to just really be messed up.
I don't recall if I heard that as theory or sci-fi, though. Ah well, me not physicist.
zen