If you text to an iPhone user's cellular phone number, they receive it as an SMS message. The AIM message on your end is simply a data transmission. If you text to another user's AIM account, they receive it in that AIM client as a data transmission receipt (not an SMS text message).
So, the AIM message you sent is included in your data plan. If you sent it to a cellular phone number, it's received as an SMS text message and the recipient will incur (or not) an SMS message charge based on whatever their plan says.
Here's a link to a MacRumors forum topic where the AOL Mac Product Lead Lee Givens addressed this issue: