So, how is it that I can get scooped by my own information? (Just kidding ZMan, I appreciate it!)
Anyway, two main known issues with the Whidbey MDX Beta. First, it requires VS2005 Beta2. Nothing later. Those of you using RC0 will get "FileNotFound" exceptions when trying to load the assemblies.
Two, there is a large 'chunk' of D3DX that is 'missing' from the assembly due to a mistake on my part. It is most certainly unintentional. (Oh, and MatrixStack is a D3DX component too, despite it being in the Microsoft.DirectX namespace).
We are working on getting these issues addressed as quickly as possible. One of the benefits of releasing every other month though is no matter what, you don't have *that* long to wait for updates. Please keep the feedback coming, particularly on design issues, but on anything you feel the need to tell me about.