problems with CHAT clients
You are right, the sidekick is a great AIM platform (and bad phone). The reason why is that it is mock-3G in its AIM implementation. You don't actually log in to AIM when you use AIM on a sidekick, you log on to Danger's Servers who run a virtual AIM client for you. That way, as long as Danger's servers are up, so is your AIM. 2G phones can do data OR voice, and so when you get a phone call, you get disconnected from the internet on your side, but Danger's servers are up, so the messages go there. Once you reconnect, the phone and the servers sync up, and you get all the messages that you missed.
I have a T-Mobile MDA right now, and while the SMS texting sucks, so do the other options. THe problem is that with a 2G phone, if you get a phone call for a second, or lose signal, you lose any messages that are sent to you until you can regain signal and get back on the internet. AIM doesn't know that you are signing off, it just keeps trying to send messages to you that you do not get. So, whoever you are chatting with can send you several messages before AIM kicks you off. You have no idea what they said, and they don't know what you missed without the 'Ok...the last message I got was ...." conversation.
This is why most phones still use SMS AIM, because you can get aims while you are on the phone or have no signal. The fact that it helps sell SMSs is a huge plus as well. I'm sure companies would sell you a AIM program for some amount a month, but unless they could make it work like Danger's version, nobody would be happy with it. I know I've hated every AIM client for WM5 because they don't work for real chatting very well, just a message here or there.
It would make sense that IF the second version of the iPhone is 3G that it would have a built in iChat client, because then the software will work well. You will still have problems with losing signal, but you can still get AIMs when you are on the phone. Hell, you will still be able to send AIMs if you are on the phone with a headset. If they were to release a non-SMS version of a chat program (again, short of setting up a Dangeresq Proxy Sever), it would do nothing more than get poor reviews and piss the customer off. Apple would rather tote the NEW ichat mobile services in the next version of the iPhone then the FIXED iChat mobile services in the next version.