It's believed it will be tackling piracy which runs rampant on the console. Reports claim an additional 1GB will be included with these new 'XGD3' discs.
Officially they say this new format will just "improve" their products and don't mention piracy at all, but apparently this new format will be XGD3 (Xbox Game Disc 3) and comes with an extra 1GB crammed into the layers. These capacity discs aren't commercial.
As part of the preview program those who get in stand a chance at a free copy of Halo: Reach and other rewards. You must be signed up with Microsoft Connect to be able to enter. You must then fill out a survey and you'll get mail telling you about the process.
summarised the program:
•We’re looking for multiple thousands of participants, so your chances of making it in are good.
•This opportunity is open to Xbox LIVE Members in US region only.
To avoid any problems, read the survey carefully and double check all the information you provide.
•While both Xbox LIVE Gold and Silver members can sign up, priority will be given to Gold members in the selection process.
Will they tell us anything more? “Unfortunately, beyond what was included in Major’s post we don’t have any additional details to share at this time," said Microsoft.