This is what I'm thinking would be nice for a Neo-Freespace skinned dialog/window (attached). Uhm, a little more artsy too.

I'm sort of thinking that this would be a good chance to get away from the possible legal problems with distributing the FS art. Done right, we could also use *one* set of art, or at least a lot of it, no matter what the interface was. And make it easier for mods to whip up their own artwork.
A lot of the work will, undoubtedly, be coding it in for the various sections.

Edit: Oh, for windows I was thinking of having the following images:
Caption background
Hide button (inactive/active/pressed)
Close button (inactive/active/pressed)
Main body (excluding borders) - different dialog sizes would be a problem.
Left/right/top/bottom borders for the body - these would extend all the way, so either the left/right borders or the top/bottom borders would have to cut off or diagonalize their edges. The chief problem is handling multiple-sized dialogs; I haven't come up with a good way to do that not involving large amounts of images. I should be able to rotate images 90 degrees without much trouble.
For resizing purposes, it would be possible to force windows to the nearest multiple of 32 or something; I don't plan on implementing user-resizable ones, as that would be a bit messy.