<p>Is there any reason that MVC isn't equally useful for desktop apps as for web apps?  </p>

<p>What would be required to make an ASP.NET MVC directory tree implementable as a desktop app?</p>

I've always thought of the term MVC as the same as a n-layer application - so correct me if I'm wrong here folks.
When i develope, I always(unless other instructed) use the following model/structure, also in applications:
GUI(Web, Winform, whatever) ->
Business logic ->
Data layer ->
And also with an underlying "Model"
... Which is a sort of MVC - So yes, it is usefull for desktop apps also. The main advantage with this, is that you can develope web, win and mobile(++) applications based on the same code.
Another thing that could be done, is to create the data/businesslayers as web-services...
I think this aproach would qualify as SOA.
EDIT:
As a note, the four levels of applications are created as seperate projects - and then used as adding reference to either the project, or the DLL, or from the GAC(or wherever you like.....) :) Thus, the need for a directory structure is not needed.