I believe there's one floating around called "aqua" but I don't think it's an OS X skin. I may be able to whip one of these up, seeing as I use it. No promises though, I'm still trying to finish another one.
I'm working on an Invader Zim skin at the moment. Just because I thought it would be cool and I didn't see any available. I'm a few backgrounds and icons short of completion. If anyone else wants to take a crack at making one, the best resource I've found for top quality IZ graphics is Room With a Moose. "The Closet" under "multimedia" has all the good stuff, even .eps files.