With a short delay to the expected date, Microsoft made available its public beta of the coming version of BizTalk Server 2006 R2.
You have to get it at http://connect.microsoft.com. Please allow some time for the downloads to show up in your instance of the connect site and remember you have to register for the beta program first at "available connections" where you should see:
Nomination Start Date
Nomination End Date
BizTalk 2006 R2 Beta
Additionally you might consider checking out those files:
Apr 03 2007, 10:47 AM