Meebo is a web-based chat client for a huge number of protocols, including Yahoo! Messenger, Windows Live Messenger, AIM, ICQ, MySpaceIM, Facebook Chat, Google Talk, CafeMom and others; and claims 190 million users.
Unfortunately they have never released a client or provided support for Windows phone 7, but with the upgrade to IE9 in Windows Phone 7 Mango this is no longer a problem, as the service appears to be working perfectly.
While the solution is not ideal, as it obviously lacks features from a dedicated client such as off-line notifications, it does cache messages missed when not in the chat window and replays them when you return, and if you just have to chat with your friend who stubbornly clings to Google Talk it does provide a work-around.
To give it a try visit Meebo at meebo.com/mobile here.
Thanks Simbadogg for the tip.