We play plenty of different RPGs, so it's hard to think of anything as "backup". If we want to shift to another game for a while, we just do so. These games include D&D 3.5, Call of Cthulhu (d20 and BRP), Mutants and Masterminds, Everquest d20 (which I personally don't like very much), Hero, and GURPS.
We play D&D most out of all of these and plan to continue doing so, most likely with 3.5 rather than 4e.
Well, I guess, I'll just continue playing 3E D&D then. I probably wouldn't play an entire campaign again, I'd play one-shots or mini-campaigns to concentrate on the level range where the game is actually fun.
Although I am looking forward to Dungeons & Dragons Fourth Edition, I have several backups in mind in case my role-playing group doesn't like the revised rules:
-True20: Solid multigenre support, might tweak combat rules (Mutants & Masterminds 2nd Ed. is already a backup as our primary superhero system.)
-D&D3.5/Pathfinder (Not my first choice, but some gamers don't want change.)
-GURPS 4e (Was my preferred game in the 1990s, but latest edition isn't well supported.)
-Savage Worlds (I and one of my players like the idea of rules-light role-playing, but I suspect this pulp-inspired game might be a tough sell to those who prefer D&D.)
-Other D20, such as Star Wars, Hamunaptra, or steampunk...
1) Pathfinder
2) Straight up 3.5
3) Some other OGL based game
4) Traveller (any version)
5) CoC
But to be totally honest, my likelihood of playing 4E long term is probably somewhere between (4) and (5) - unless it's a very different game than what's been revealed to date.