I'm in the process of developing and releasing an iOS game, and have found that it just won't run on an iPhone 3G (don't even ask about a 2G) for many reasons. Primarily, the number of sprites and sounds needed for each level continually cause low memory crashes. I've already pared it down to the bare minimum and strategically load/unload resources as needed, but some items need to be cached so they can be activated quickly. My last resort (which I'm trying to avoid) is to make lower quality versions of all audio. Graphics are already packed tightly into sprite sheets.
The game runs wonderfully on an iPhone 4 and iPad, and even performs nicely on a 3GS. Should I be concerned about no support for older devices?