Here's something cool I discovered, purely by chance. In the message-compose pane of an Adium conversation, you can 'page back' through your composition history with (on a Mac) 'option-up arrow.' I would assume that the Windows equivalent is Alt-UpArrow.
I guess a good question would be 'Why would I want to do this?' (After all, how often do you repeat statements made on IM that you've already made within the same session?) It's a nice little easter-egg interaction, but it seems like a carry-over from a command-line-interface mentality.
Actually, I just thought of one scenario in which this functionality would be very handy: if you interact with any AIM (or YIM, or MSNM, etc.) bots on a regular basis, it might be nice to repeat commands, varying them by an search-query, or an attribute or the like.