No one can promise that AIM will always be free. That depends on a big company called America Online. And AOL is always looking for new ways to make a buck.
Technically speaking, AIM is not “free” now. It is paid for by the companies who pay AOL to show their advertisements in the buddylist and chat rooms.
But to answer your question more directly, I suspect that the basic AIM program will always be free to download and use. You may have to pay for extra features, like AIM Expressions, but since all the other big IM products are also free (MSN Messenger, Yahoo! Messenger), AIM will have to remain free in order to maintain their market share.