FWIW, did you know that AIM is also a text messaging gateway for the US and Canada?
Send an AIM message (using iChat, AIM, Adium, Proteus, etc), entering the target cellphone number in the form +1xxxxxxxxxx where the x's are the 10 digit phone number without dashes or other separators. The recipient will receive a text message that will indicate the AIM user ID of the sending party and also allow you to reply by text message (the reply text message will appear as an IM).
Even though you can / must (I think) use the country code, AFAIK the service only works with US/Canada numbers... 🙁