I've thought about this a little bit, and this by far seems like the most likely reason: You can't receive calls and use EDGE at the same time. Apple is not really publicizing this, because most of the time it isn't very important: you only pull out the phone to look up directions or search the Internet once in a while. But people want to leave their chat clients connected all the time. Unfortunately, you can't receive calls if you're logged into chat over EDGE. So it'll always be one or the other: you can either receive an IM, or a call, but not both. It's simpler for Apple just to eschew iChat this time. Less angry customers, and they'll be able to implement it and sell it as a new feature once 3G is widespread enough in the US to be cost-effective.