In addition to the LIVE Messenger integration feature in the new Dashboard, you’ll also see:
* New achievement pop-up with name and score value
* New Marketplace blade places content all in one place
* Friends playing now feature at Xbox LIVE Arcade
* Console can be set to turn off after downloads to save energy.
Microsoft Press Release
A good update and much needed I think. Still no DIVX or XVID support though :-\