Longhorn is being showed off at Winhec. It's still over 2 years away so much will happen between now and then.

From a customization point of view it's going to be exciting though fundamentally different.  Check out the screenshot included here to see what I'm saying.

This build of Longhorn still includes two of the old fashioned techs for doing visuals (Jade and Slate).  It's the Longhorn style that shows off what is possible. So what does it mean for skinning?

In a nutshell, customization (Based on the current direction in Longhorn) will have to deal with the client and non-client areas seperately. There's no "uxtheme" type stuff here, this is all handled by the compositer (though like I said, this build still includes the old type of visual styles which are being phased out).  Customizing the non-client area will be interesting depending on how its implemented. 

The client skinning portions, by contrast, will likely be more like traditional skinning has been done in the past for compatibility reasons (XAML based apps though are still up in the air).

More to come shortly.