Not a dev (just a tech nerd), but afaik ARM and PowerVR stuff is good about backwards compatibility, but you might run into issues with the OS versions. Like if you require Game Center (or some other 4.x feature), it won't work on iOS before 4.x, and iPhone 3G. There are also some older games that haven't been updated that break on 4.x for whatever reason. There might be particular rules you have to watch out for but I'm not sure on that, just read the agreement carefully I guess.
The only differentiation I've seen on the store for the most part is "HD" versions (which sometimes includes the iPhone 4/iPod touch 4th gen), not sure I've seen anything separate for way old devices like the 3G or older OS releases. It generally seems to be a choice of supporting older stuff with the same release or not at all. Probably don't want to spend extra resources on something that isn't going anywhere.
Good to know...still I'll probably hold off until it goes on sale. Perhaps a "shamrock special" sale in March, or an "easter extravaganza" in April, or just a fire sale for iPad apps to prepare for the next iPad launch. Anyone tried playing it on an iPad yet? I'm sure it looks like ass, just wondering if the controls work ok scaled up.