This is the Windows XP "Zune Theme" for Metacity. As far as I've found, I'm the first one to port this theme over to Metacity (or, to any Linux window manager theme period, for that matter).
When you maximize a window, the titlebar loses its round edges and borders ("a must for any Metacity theme").
Update 0.02 | Oct 12 2007
I've added an XFCE window manager theme for Zune in here as well. If you use XFCE or even just use its window manager, you can use the Zune theme on that too!
(I use the XFCE window manager on GNOME because it has compositing, which Metacity lacks. Info here: http://forums.fedoraforum.org/showthread.php?t=163632 )