In OS X Lion your scrollback is maintained between subsequent launches of the Terminal, especially noticeable between reboots when "Reopen windows when logging back in" is ticked. I mentioned this because a very similar issue, see: http://superuser.com/questions/315174/how-to-open-a-clean-te....

This is specifically a flaw in the libVTE terminal library, which OS X's Terminal.app is not, to the best of my knowledge, based on.