I saw over at Pete Cashmore's Mashable that Microsoft and Yahoo to integrate IM services, FINALLY. Its still not open, but at least some progress has been made and MSN Messenger users will be able to talk directly to Yahoo Messenger users.
As for those apps that let you login to multiple services simultanously, I did try out Meebo the other day, and it's pretty good. I tried Trillian but it was too buggy.