Linked to Defcom's Post to follow on.
There is also the way you can get to call back the previous or part of the Last chat you had with that person.
If you tick the box I have marked and then chose one of the options in the indicated drop down you will see either the whole of the last chat or the last x IMs of that chat when you click a Buddy to start a new Chat.
It does take a while to build up a history. (mine goes back to 2004 in some cases).
However it an be useful also when someone contacts you as this will trigger the setting as well.
iChat 4 does not have this setting but has a different one that keeps the Last Chat (with whoever) open across Shut down and restarting iChat.
With a good history of chats iChat 5 can run a close approximation of this.
iChat 6 will reopen any window that was open when your Quit iChat.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously