well..... actually fallout kinda ripped off (or at least was greatly inspired by) a game called wasteland (arguably the first truly post-apocalyptic RPG).
thing is, fallout really jumped on the 90s upgrade in home computing and processing capabilities as well as the fact that it had better writing (or at least a dark comedy setting) compared to wasteland which was a straight/serious rpg.
don't get me wrong, i loved fallout since fallout 1 and i can't seem to get far in Wasteland 2 - i really suspect this is mostly cos of the comedy factor