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Im not exactly sure how to do this, but someone mentioned Chatting on IM using Text messages, and since you have unlimited, it wouldnt be too bad. Let me try to find the link...

I do this already. When I'm logged out of AIM, IMs are forwarded as texts and I can respond. Problem is, I can't see who's online, so I can only reply to existing conversations, not start new ones.
 
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