Microsoft has released the developer edition of BizTalk 2006 R2 on MSDN. Along with Windows 08 RC0.
Anyone with MSDN access should now be able to access the software. Non-MSDN users can still access R2 but its a free trial version.
I'm not sure what the plans are for posting other editions on MSDN or what the time frame is.
Happy BizTalking!