I recently submitted the Design Guidelines for Exposing XML Data as part of the WinFX design guidelines. You can read the guidelines in Krzysztof Cwalina's weblog. These are also the application guidelines that developers working with XML should follow for working with XML in the Whidbey timeframe. I'll be working with the FxCop team to get some rules written to check for compliance to these guidelines in the Whidbey base class library over the next few weeks.