I've checked iChat's built-in help, as well as Apple's site and couldn't find anything that has mentioned this. In Leopard, iChat has a sort of built in "answering machine" that I hadn't noticed before. It works like this:
Find a person on your Buddy List who is currently offline. Double-click on their name to initiate a new text chat. Whatever information you send to them will be presented to them at the time of their next login. In previous OS versions, attempting to instant message someone who is offline would result in an error message. However, 10.5 allows you to "leave a message" without a problem.
I tested this using AOL Instant Messenger accounts, and going from Mac-to-Mac (using iChat as the only client on both computers). I have not tried this using other IM accounts such as Jabber, GTalk, or .Mac, and have not tried this with any other IM software on either Macs or PCs.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080127033009691