In the meantime, check out this latest App Hub release: http://create.msdn.com/en-US/sample/xnb_format
This article documents the format of the compiled data files (.xnb) produced by the XNA Game Studio 4.0 Content Pipeline build process. This information may be useful if you wish to write your own code to load .xnb files into a programming environment other than XNA / .NET, or if you wish to create .xnb files independently of the usual Content Pipeline build process.
It also includes an example .xnb parser, written in native C++, which demonstrates how to parse a compiled XNB file by printing its contents to the screen. This can be used as a reference to help you understand how to write your own XNB loader, and also as a debugging tool for investigating exactly what data is contained within an XNB file.