Just a quick note to say that iChat logs includes images the images you send, so they can get pretty big if you and a friend are comparing holiday photos via iChat like I do.
[robg adds: Logging in iChat is turned off by default; to enable it, open iChat Preferences, click on Messages, then check Auomatically save chat transcripts (and optionally pick a folder). Once enabled, things do tend to pile up relatively quickly, and images are indeed stored with everything else. I'm not sure about attachments, though.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040520172410950