Currently, it cannot.
However, in June when the free iPhone software 2.0 update will be available along with the AppStore (this update will cost a "nominal fee" for the iPod Touch), AIM should be available.
However, I do not know how much AOL will charge for this application, hell, it might even be free. But if you watch the March 6 Event for the iPhone (apple.com) you can see that AOL managed to accomplish a decent version of AIM for the iPhone in just two weeks.