No way Apple makes you use SMS for iChat. Though iChat, which Apple is sure to release for the iPhone, only supports AIM.
The reports say that Apple went into the negotiations with providers basically saying "you aren't putting your crap on our phone.... but we'll give you a 5 year exclusive." And they still got turned down once. I really doubt Apple will allow ATT to stick anything on there.I don't know, we'll see. Again, AT&T released their own chat app for Treo that does use SMS, so you get billed for every message.
On another forum the rumor was it had something to do with the leopard timetable, but who knows.
ps: also, it's not like there aren't several (albeit annoying) workarounds. And I've been using my treo 650 without IM for years. Though I would hope apple will fix this soon, and support networks other than AIM (jabber would be great).
I believe the leopard issue had to do with actually using the same code for both, I simply don't recall. I'd rather that they take some time and do it right; phone seems buggy enough as it is. And I'm not sure this is as critical a feature as you seem to be implying. I personally don't know anyone who does IM on their phone, though I'm sure lots of people do. But that's still a lot of customers who can live with email and SMS.
I'm not sure i know anyone who would consider it a critical app as most people dont have it on their phones at all. There were never any good, free IM apps for Palm OS so i rarely used the one i had. It wouldn't surprise me if we saw an iChat client that supported both AIM and gtalk which is all anyone i know uses.I think many people find IM a critical app. And nearly everyone I know uses IM on their smartphones/PDA.
Anyway, I'm sure they will come up with something.