I had no idea this was possible: I just clicked on a link on this page -- uncharacterstically, without checking the status bar first -- and next thing I know, AIM had launched and sent this girl a message (the message being, "abi,that's disgusting"). Yowzer. I exited quickly. That's a bit more personal than I wanted to get just then, having only landed on her site for the first time about 30 seconds prior. Still, it's kinda neat. Of course, I'm an AIM neophyte, so perhaps everyone already knows this, but if not, this seems to be the syntax for the HREF:
aim:goim?screenname=screennamehere&message=entermessagehere