The New Xbox Experience has been officially released to all Xbox 360 owners, although it may take some time before your own console can access the update. According to Major Nelson it may take anything up to a few hours before Xbox 360 consoles are offered the new software, as the switch on process is seemingly staggered rather than simultaneous.
The update adds Avatars, Themes, Marketplace on Xbox and Netflix streaming movie functionality, among other things, as well as a fresh new 'blade' interface that should be easier for Microsoft to add new functionality to in the future. However the Netflix launch is currently mired in controversy, as licencing issues between a Sony Pictures subsidiary and the media distributor have seen several hundred titles left unable to play on the Microsoft console.
You can see our preview of the New Xbox Experience here. The update is free, occupies around 6GB of your 360's hard-drive, and should be available sometime today.