hey JJenny!
When you have AIM forwarded to your handset if does come in as a text message.. and will count toward your txt usage.. but there is a way to avoid that. Your phone should have AIM included on it (most do) If you connect via the little AIM app on the phone its like using the internet to connect. it uses
GPRS to let you chat. This is the same thing that makes T-Zones work.
IF you have added the T-Zones to your account you have nothing to worry about. its not calculated as time used. You can get unlimited T-Zones for $4.99 a month. T-Zone usage is not calculated as text message time.
T-Zones uses something called
WAP or wireless application Protocol.. txt uses SMS (simple message service)
If you do not have t-Zones aded and you would like it.. you can call customer care.. or add it your self at mytmobile.com
Hope that helps