According to Microsoft the new release wil focus on 3 main themes:

• Platform Support – Supporting new and current server and development platforms
• Improved B2B – Aligning with industry standards and continuing to invest in performance improvements
• Ready for the Cloud – Enabling customers to take advantage of the benefits of cloud computing

BizTalk Server 2010 R2 will be delivered approximately 6 months following the release of Windows Server 8. Release timing for Windows Server 8 is yet to be announced.

More info: BizTalk Server 2010 R2 – BizTalk Server Team Blog