I found this link over at NeoWin. They claim this site offers beta subscriptions to Microsoft's upcoming Mail.
We think you're going to really like the clean new look and other features -- at least we hope so! We need your feedback to make sure we are doing this thing right.
To sign up for the beta, please give us your e-mail address, and check the box if you want us to keep you in the loop about the latest developments. We'll use your e-mail address for this beta release and relevant updates only.