AOL has added a new feature which allows you to use iChat on two computers at the same time! If you're online on both computers, they will both get new messages. If one computer is away, then other online messages will only reach the online one. I use this to have my home computer as a communications center. It's always away in iChat and I can connect to it using VNC to review older messages.
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.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003112714581660