I agree with Backblast - the way to work in comebacks in CIV 4 is to program the AI to go for a cultural or diplo victory when it (the particular civ) realizes that its not going to win any other way. That's my usual comeback strategy at least - I've won a bunch of games through cultural victory where the race was against another very dominant civ going for space or domination. It can be an exciting way for the game to end. But you never see the AI pulling this off.