This is the tool I made heavy use of at the XNA Users Group last
night:
http://www.red-gate.com/products/reflector/
Reflector is a program that lets you examine and disassemble any .NET
DLL or EXE file. I used it to disassemble the XNA DLLs into C# code
so that we could look at what XNA is really doing behind the scenes.
The program is free to download.