MacMinute notes that AIM version 4.6 for the MacOS adds SMS support. We noted last week that Apple's iChat application has the same technology built in. To send an IM to a mobile phone, send it to +1xxx-xxx-xxxx without the dashes (just used to clarify).
The latest Windows version also includes this feature. It's not yet clear if all carriers support this, but we have confirmed that Verizon, T-Mobile, AT&T Wireless, Nextel (thanks Andee) and Sprint PCS do. If you know of another carrier that does, please let us know.