A slightly modified version of the theme found here http://gnome-look.org/content/show.php/OsX_MoD?content=54851
I take no credit whatsoever for making it. The only changes are that there is an iTunes icon for the music player Songbird and this http://taimila.com/downloads/apple.png is used instead of the Apple logo actually used in Macs, because I think it looks better.
For screenshots, go to http://ryanlesterstore.googlepages.com/ubuntupwnx0rs
For an awesome guide to theming GNOME like OS X, go to http://taimila.com/osx-guide-2.php