Johan,
Visual Studio 2008 doesn’t support the btproj project type so no support for BizTalk 2006 just yet – you’re going to have to keep that VS2005 installation hanging around for a while yet.
However with the release of BizTalk 2006 R3 yesterday, VS2008 integration is just around the corner – see Steve Martins blog: http://blogs.msdn.com/stevemar/archive/2008/04/23/biztalk-server-platform-updates.aspx
Nick.