This applies to those using the ever-popular IM client, Adium.
Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
If you are in a situation were you would need to back up your chat transcripts due to account migration, or in need to do a clean installation of your OS X machine, here’s where to start:
~/Library/Application Support/Adium 2.0/Users/Default/Logs
Note: This is current as of Leopard OS X 10.5.4, and Adium 2.x.