Cool idea. May I suggest that you make it so people can make themes for it? Maybe make it a framework people could base their application out of, or even make some Interface Builder plugins, geesh I dont know but this is certainly an interesting idea.
One nice thing would be to make it very easy for someone to make new themes, and use XML based theme plugins. Maybe even place a themes folder under /Library/Application Support/Themes or ~/Library/Application Support/Themes or /Library/Themes or even (if you're wild enough) ~/Library/Themes
I guess I need to think about this idea a little more, to offer more suggestions.
