Well, i can see next gen and pc because you can require pc's video cards to be at the nex gen level or higher and that'd work out easily enough.
Trying to go back to 360/ps3 would be difficult if they're writing to the video capabilities of the new consoles. They'd have to likely give up a lot or completely rewrite the hardware acceleration/dx routines for the lower end chipset. PC video cards from 3ish years ago are still as good/better than next gen consoles. The hardware in the the 360/PS3 is 7 years old, and was comparable to what pc video cards could do 2-3 years prior to that. They were able to go as long as they did and as quality as they did because they could continually write for a single chipset and single set of abilities and keep pushing and revising it. I mean, i think the chipsets of the last gen were based on geforce 7000 series cards. Since there were 8000, 9000, 200, 400, 500, 600, 700, and they're about to release the 800's...