I've done it a few times, using the chat system. Other than using the chat as something to do between sets, it seems relatively pointless. After all, you can't really "spot"someone over the internet. Other guys have been on chat while on cardio equipment. A drawback is that if you're using the audio link, rather than the keyboard, you can only hear one other person at a time.
From the inevitable IM requests to "take your shirt off," it appears that some regard this as a form of soft porn. I have nothing against that really, but if you ever try to do decline presses on a sweaty vinyl bench, it becomes clear that it's not very practical.
With the spread of urban blanket WiFi and iPhone-like proliferation, I suppose this will become just one more annoyance in gyms.