Microsoft is looking for a few willing participants for their new Xbox 360 System Update Preview. By "a few" they mean "multiple thousands," so you actually have a decent chance of making it in. Basically, participants will be previewing updated Xbox 360 Dashboard features. You can see all the details over at Major Nelson's blog, or you can go straight to the sign-up via this link. Participants will receive a copy of Halo: Reach for their troubles, as well as "other possible rewards."
There are a few caveats. You need access to Xbox LIVE. Both Gold and Silver members can apply, but Gold members will be given priority. And finally, this is open to Xbox LIVE members in the U.S. region only. Judging by the comments on Major Nelson's blog, you're best bet is to use Google Chrome when filling out the application. It takes about five minutes, and you will need your system information to complete the form. Good luck, and if you get selected, enjoy your free copy of Halo: Reach.