This one is actually a modification of previous ones I experimented with. The taskbar isn't my work, it's from the Kore-Gnome theme: http://gnome-look.org/content/show.php/Kore-Gnome?content=5574, which is also the GTK theme included in the screenshot.
The window is just Thinblack modified with mac-ish buttons that glow blue when you hover.
Wallpaper is included in theme, but compressed. Here is a better version:
To change the background, edit the last line in the Default.theme file to point to the background you want.
Update 1: Fixed a minor error with dialog boxes.
Update 2: Improved the buttons
Update 3: Improved the buttons, again
Fixed one of the taskbar buttons to show the proper background
Update 4: This is a major update
-Fixed error with dialog boxes not showing properly
-Changed the taskbar significantly (Uses the taskbar from the Cobra theme)
-Changed the menu and start button as well
-Now uses the Zekton font, make sure you have it installed