There is a sample Chat Loggin applicaiton in the default installation ( since 6.51 at least ). It can log all the content of 1-1 or n-way chat to files. In Sametime 7.5.1 we added a functionality that the snapshot pictures sent between users are also saved.
Please look at:
http://www-1.ibm.com/support/docview.wss?rs=0&uid=swg21086729