Microsoft is currently seeking "multiple thousands" of testers for an Xbox 360 system update preview program. Unlike previous previews, which were for upgrades to the 360 dashboard, this one includes physical items being sent to participants -- it's intended to help Microsoft "prepare for an updated Xbox 360 disc format."
No further details were offered about exactly what the new format is all about. Things will be kicking off in the next two weeks, so those who apply should be receiving a notification by email fairly soon if they've been accepted. You must be an Xbox Live member located in the United States in order to participate; being an Xbox Live Gold member will give you a better chance of getting in.
Those who are accepted will receive a copy of Halo Reach "along with other possible rewards." Interested Xbox 360 owners can apply for the preview program here (http://connect.microsoft.com/xboxliveupdate).