They also have to support PAL if they're shipping outside of the US. Then there's 1080i and 480p or whatever the HD resolutions are. You have to make sure that the game doesn't crash on those.
It's not a matter of the silicon driving it, the developer has to support it.
And no, their input is not locked to 1/frame. Unless they choose to do so in their game (or possibly previous 2 generations, but current ones are not explicitly locked).
Best solution is to just develop for PCs j/k. No, what you said at the end is perfectly accurate. Keep the FPS constant (not just a constant average, don't let the min/max vary by more than a couple frames. people can tell!) and don't let objects move too fast. Or if they do, add some motion trail (blurring generally looks stupid if the rest of the world is crisp) so there is *something* To fill the gap.