Nuke: That looks good, but in theory, all of the dark and light red bits could use one and the same file, and the blue corners too, right? The caption and status bar could even be the same file(s), by the looks of it.
Now, don't get me wrong, I'm not saying they should be seperate files, but that if a mod would want to do interface art quickly, they could be the same image. This'll leave you with 4 files, assuming you make the end of the caption/status bars the same image mirrored, and all the sides/corners are rotated to fit. You then only add the images for the buttons, which can go from 1 to 9, according to state and amount needed.
These requirements sound very nice, because it'll allow mods to be reallly quick with the skinning. Now, if you make a table to allow for seperate images for each of the regions as well, you'd get a best of both worlds situation, allowing one to make each image special, for a maximum of 20 files.