I am using the AIM v2.0.46 on my Curve 8320 with a 4.5 OS. I am able to send text messages via the AIM program, which uses data to send it. What I do is start a new conversation, and put the contacts number with a +1 in front of the number (i.e., +15555555555). Seems to work just fine.
The 2.0.46 version of AIM comes with the 4.5 OS, I believe.