Howtoforge posted a guide about making your linux desktop look like a Mac

Do you want to give your desktop a dash of Mac OS X? The goal of this project is to bring the look and feel of Mac OS X (latest being 10.5, Leopard) to *nix GTK based systems. This document will present the procedure to install Mac4Lin pack & tweak certain things to get that almost perfect Mac OS X like desktop.

Make Your Linux Desktop Look Like A Mac - Mac4Lin Project Documentation