Do a search of IM and SMS on this forum and you will find several threads that discuss this. Usually it's folks looking for free IM or IM over SMS only which doesn't exist. Check carefully as I think, while I'm not sure, that Aim and Yahoo use SMS to send messages but Data to log into your account.

p.s. Last threads I saw say you can't use AIM on AT&T so you would need something like Jivetalk.