BizTalk Server 2006 Beta 2 is available for registration and then download.

1. Go to http://beta.microsoft.com and log in with your MS Passport account.
2. Click on "Sign in as Guest" and use this Guest ID: BizTalkBetaTeam.
3. On the left side, under "Program Info," click on "Survey" and complete the nomination form, and submit it.

You should subsequently receive a confirmation email with further information about downloading hte beta, newsgroups, etc.  After you've received that letter, you'll be able to download the good bits.