The XNA Games Studio Express beta is available. It's not a complete representation of the final Games Studio Express functionality, but it's a good way to get acquainted with the style of the thing.
There's a lot to go over, even for Managed DirectX veterans. You'll notice a lot of name changes. You might find that there are a lot of new, unfamiliar technologies available. You will almost certainly find that some familar Types and Members are simply no longer present or have been replaced.
I talk about a lot of this stuff in my upcoming API migration whitepaper. A Beta version should be online soon. When it goes live, it'll be linked from the XNA Developer Portal. That should answer some of your questions when you start porting MDX source to XNA. As soon as it is live, I'll post about it here.
I will be on the forums a lot in the next couple days, collecting your feedback and answering specific questions about the XNA Framework.
Update: I wanted to link to an excellent article on David Weller's blog. How to report XNA Framework bugs to us.