When I'm at the university I launch iChat on my PowerBook and get new messages there instead. I haven't tried if this works on more than two computers.
[robg adds: This is definitely a new and nicely appreciated feature; it used to be whenever I opened my laptop, my desktop iChat would disconnect. Testing it now, they both stay connected. It's a bit odd, as the laptop won't see what I type in the desktop chat window, but it will show the other participant's response there.]

