BizTalk 2006 Beta1 is available...

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... from https://beta.microsoft.com

If you are not registered there, you'll have to pass through sign-off approval process first, because that's the generic place where Microsoft products betas are available.

Notice that once you logged in with your Passport ID, you need to use (on the second screen) GuestID BizTalkBetaTeam to get straight to BizTalk 2006 Beta 1 form. If you use the Guest ID published on the passport login screen of the Beta place, it will take much more steps to get there. More complete instructions were posted in the Beta newsgroup.