Sci-fi settings where the Earth, the home of humanity, is a lost/forgotten/forbidden place: GO.
Off the top of our heads:
Dune
Titan A.E.
Battlestar Gallactica
After Earth (the movie trailer we just saw with The Smith Twins)
NB: Doctor Who does NOT count even if he's spent many episodes based on post-destruction Earth humans, because he's constantly going back to Earth. In the past. Bloody time travellers.
Source:
http://theweaselking.livejournal.com/4347724.html