Dexrex is the free add-on for archiving IM & SMS text. Sign in with your screen name, download the plugin, and start saving your world one message at a time.
Based on what I looked at, it would appear that your saving to their site. I didnt play with it enough to fully understand where IM chats were being logged too.
You got a little further with this app than I did. The thing wouldn't even install and run for me, it showed up as an icon on my homescreen, looked a little like a command line window icon in Windows.