There's a new white paper available on MSDN which provides testing guidance for Microsoft Dynamics AX ISVs. Also available is a simple Excel based template for test case management and a sample test case created using the Task Recorder feature in Microsoft Dynamics AX 2009. All three of these can be downloaded from http://www.microsoft.com/downloads/details.aspx?FamilyID=ec0c1650-ef44-45e9-ade9-57f28c39a051&DisplayLang=en.
I was responsible for most of the content in the paper, so please provide any feedback that you have to me. There's a large demand for testing guidance in the Dynamics ecosystem. This document is a start, but I know there is more work to do. I would love to hear some feedback on this document and what else you would like to see from Microsoft in this area.