I am not sure if anyone else has noticed this yet, but before with Media Center Extender with the Xbox 360, you could not use the MSN Messenger capabilities like you can on the PC, until now it would seem.
With maybe the factors of Windows Live Messenger Beta on the PC and possibly even the March Xbox 360 update, you can now access MSN contacts and chat through the Xbox 360?
Also seems upon further inspection that there is no USB keyboard support in Media Center for the Xbox 360 yet, maybe why no official announcement has been made. Quite tricky using the remote to say the least.
[UPDATE] Xbox.com MVP posted in my blogs comments:
The windows messenger inside of MCE has always been accessible via the MCX (Media Center Extender) for the Xbox360.
There were no new features added, all it did was make the ‘user experience’ better by fixing some little backend stuff.