is Longhorn Build 4015 desktop based on XML?

From what I understand the file system is based on XML. Other than that, I'm unsure.

nSnaveZ: ope, only the sidebar, though longhorn will include a new language called xaml (similar to xul).

Mear|in: filesystem is ntfs, but it includes a db-like storage engine based on sql, running as a service.

you can find more info here: http://longhorn.msdn.microsoft.com/portal_nav.htm