Infosys have just published a number of books and whitepapers which cover the key features in BizTalk 2006, VSTS and .NET 2.0 on their website. Complimentary copies of the books are available, the whitepapers are free downloads (PDF).
Of particular interest are:

BizTalk Server 2006 (book), which ‘provides a critical evaluation of the new features of BizTalk […]