Here. John Carroll says it nicely, I think:
"Longhorn will be the first operating system where ALL functionality is designed to be accessed through managed code. WIN32's reign as the Windows API has ended, replaced by managed .NET APIs. ... WIN32 will still exist for backwards compatibility, of course, and native access APIs will exist for those applications which need it... However, Microsoft intends to ensure that all Longhorn functionality is accessible from a 100% managed program."
He goes on to say other things that I think are interesting for the Longhorn developers of the future to know.