It will be more usefull in Longhorn and Avalon sepecificaly because you can define you UI part of your class in a XAML file and the code in the other part of the class in a CS file (in C# for exemple).
But I'm sure programmers will find other creative ways of using it .
I'm just curious about what language are the "pillars" written in. Avalon, Indio, WinFS are exposed through WinFX. Since WinFX is a .NET assembly, can we assume that all the pillars are going to be written in C# ?
Well they are for a big part in manage code, and MS just love C# so we can assume they are written in large part in c#... For source, look on this site and the .Net Show (https://www.msdn.microsoft.com/theshow/)
And I suppose that the avalon and indigo part for XP will be release at the same time that Longhor beta