You don't need to Jailbreak. You can email SMS's. It gets sent as an SMS, but is free for you and if your friends reply to it it's free for you again. AIM apps are in the App store, but you need to have the application active in order to receive any messages. I'd get a MobileMe account as it alerts you instantly when you get a message. I'm not sure where you're located, but it gets you unlimitted SMS (basically) for $100, where as if you paid for unlimitted SMS it'd cost you $240 per year.
However it is a little work to put in all of your contacts numbers as emails. and you have to know what provider they have. For example someone on the verizon network's email will look like this
5555551234@vtext.com