Following on from my previous article, one of my old colleagues has written up an article about how a project he worked on was able to simulate the behaviour of Dynamics AX by using one of the approaches I described in my article about how to simulate external systems in testing.
Check out Charlies article here: http://geekswithblogs.net/charliemott/archive/2009/05/13/stub-of-dynamics-ax-for-biztalk-development-and-bizunit-testing.aspx