Sorry, I don’t have the System.Exception right now, but it basically said that BizTalk did not receive a response from the Web Service, and the error was in System.Model…

I went to the IIS server, and browsed the System Event Log and found this message:

System EventLog (not Application EventLog):
Event Type: Warning
Event Source: W3SVC
Event Category: None
Event ID: 1011
Date: 12/1/2008
Time: 3:29:06 PM
User: N/A
Computer: S-SV-ES01-WSV01
Description:
A process serving application pool ‘DefaultAppPool’ suffered a fatal communication error with the World Wide Web Publishing Service. The process id was ‘19164’. The data field contains the error number.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Data:
0000: 8007006d

Googling on the code above 8007006D indicated that the application pool was crashing.

Eventually, we discovered that we needed to install .NET 3.5 SP1, and it resolved this problem.

This site provided some info, but turned out to be a waste of time for our specific scenario:
Blogs.MSDN.Com (David Wang)