Steven Martin posted on his blog that the next version of BizTalk Server will be BizTalk 2006 R3. You can get more info from his blog. But it doesn’t seem like the BizTalk vNext that people talk about with integration of Windows Workflow Foundation. That could just maybe happen in Oslo timeframe?
Will be released in calendar year 2009 and since it will support .NET Framework 3.5 I guess development will be done on Visual Studio 2008.
Those keen to check out how latest version of BizTalk helps to realize the SOA vision can apply for beta testing at http://connect.microsoft.com