This is not as much a hint as a bug, I think: iChat stores files you transfer and/or receive in its logs, which has the potential to lead to enormous log files. I realised this when I saw a backup that is usually a few MBs suddenly was several hundred MBs. I filed a bug with Apple on this (#4133542) issue, but they closed it with the explanation that it "behaves correctly."
If you don't agree with Apple on this, file a bug report. However, whether you think this is a bug or not, backup routines might need to be modified to handle this, especially if you pay for your own backup storage. If you find that your backups are taking much more space than you think they should, check the iChats folder in your user's Documents folder.
[robg adds: I think I'm siding with Apple on this one -- since the purpose of the chat logs is to look back at what you've done, it's nice that everything is there. However, the advice about checking the size of the chat logs is good. My iChats folder currently contains 4,581 items, taking up about 400MB of drive space. Note that logging is not enabled by default in iChat's preferences.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050823111901128