Here is the offical announcement from xbox.com:
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http://www.xbox.com/en-US/community/news/2006/0130-autoupdate.htm
This free Xbox 360 update is great news! We've been listening to what the community has been telling us, so we're happy to announce that this update will improve your gaming experience.
Here are some of the changes in this update:
-Improvements to the Xbox Guide.
-Option to retain your saved games when you delete a profile.
-Increased accuracy of "last time played."
-Network configuration improvements for Xbox Live members in the Netherlands.
-More detailed messaging for unreadable disk or region errors.
-This free update is available to all Xbox Live members. Your console will be updated the next time you sign in to Xbox Live.
UPDATES:
The update will be applied whether you have the Xbox 360 or the Xbox 360 Core System.
Per Major Nelson: If you are not getting prompted for the AU, then test your connection to live (System Blade->Netowork Setting->Test Xbox Live Connection) and youll get the prompt.
Per XeZaR: The update will be soon available on Game Discs and the OXM Discs.