OK, so it looks like someone has put together a tool for unit testing BizTalk Server.

Wow, I wish I'd know that this existed on some of my previous projects.  I'm going to have to go and play with this now and see just how good it is.  I normally end up writing a whole heap of extra BizTalk stuff (i.e. pretend SAP systems etc) to test my solutions, I hope this will make things quicker and easier for us all.

You can get more information on Kevin B Smith's blog. 

Worth a read I feel