If you want a chance to get a early look at the upcoming Zune, Facebook, or Twitter features for X-box 360 then step right up. Major Nelson announced on his blog that all X-box LIVE members (including all regions that support X-box Live) can sign up to try to get into a preview update.
All you have to do to enter is fill out a survey for them, but not all entrants will be accepted. If you are accepted you will recieve an e-mail in about a week confirming that you can join in the preview update. X-box LIVE silver members can join in too, but Gold members will be given priority.
You can find out all the details on how to enter here.