From Thom who got it via Steve
How to get BTS2006 Beta 1:
Within 1-2 business days you should receive a confirmation letter from Microsoft welcoming you to the BizTalk Server 2006 Beta Program. This letter will contain additional information on where to download the Beta and access the support newsgroup.
And if you're using BizTalk 2006 for a US Federal Government project, please contact us...we'd love to know what you're doing with it and if we can help. Thanks!