Earlier today we saw a tweet that inspired this post. Patrick Burleson asked how he can stop iChat from automatically joining chat rooms he visited during an earlier session. It's possible, but you won't find the fix in the preferences.
First, I'll describe how to join a room automatically whenever iChat is launched. From the File menu, select Go To Chat Room.... A window appears. From there, select the correct account and add your room to the Room Name field. Next, click the "+" in the lower left-hand corner to add it as a favorite and select the "Autojoin" check box. Now, iChat will join that room each time it's launched.
To disable the autojoin, simply de-select that box. Why would a person want to automatically join a chat room, you say? Some folks collaborate with far-flung coworkers via AIM all day.
There you have it, Patrick (and TUAW readers). I hope this helps.