I've been working on this for a few months since I got my Surface RT, and realized there wasn't a solid gtalk/gchat app. It's a C#/XAML app, and I ended up writing my own XMPP stack on-top of raw sockets since there wasn't a good open source one available (that would run in RT). I'll open source my XMPP library on GitHub shortly once I've cleaned it up a bit.Check it out!
Store link:
http://apps.microsoft.com/windows/en-US/app/gabberwocky/2d7a19c5-2522-461c-8706-99ff59bd84fe
Facebook:
http://www.facebook.com/gabberwockyapp