We have been collaborating with  regional director Morten Abrahamsen (aka Morty) on an Indigo Adapter for BizTalk Server.  He made the first snapshot publically available on his blog here.  Currently working functionality includes: TCP, HTTP, Named pipes, basic transport (request/response and combinations), Windows Security, and reliable messaging.    Note this is for the adventurous explorer only at this point and please do read the disclaimer below very carefully:

N.B. The adapter is NOT supported, and will NEVER be supported on BizTalk Server 2004 because it requires an unsupported configuration for BizTalk Server 2004, namely running on the .NET Framework 2.0.  BizTalk Server 2006 will support running components on the .NET Framework 2.0 in process.

We will continue to iterate on the Indigo Adapter as Visual Studio .NET 2005 continues through beta 2 and Indigo drops new public bits become available.  Morty is doing lots of great work here, and I'm thankful for it.

One final point if you want to download the whitepapers from my post yesterday you need to first join the msn group (groups.msn.com) called biztalkserverstuff.