I too have had this lost password problem.
May I make a suggestion here?
Allow the rightful owner (which can be identified by the address in the chat log and the MSN log in?) to override this with his/her current password. What I am saying is that if the user indicates he/she has forgotten the old password to the chat log, allow him/her to decrypt the file using his/her current password?
Just a thought. It's very irritating to not be able to go back to one's old chat logs. And yes I should have remembered the old passwords.