An excellent guide on modding Ubuntu to completely look like Mac OS X. Dock, icons, fonts and all! By the designer of the Orange Look theme. So, it you are the one who wants their desktop look like Mac without using OS X, just go ahead and read full acticle.
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Make Ubuntu look like OS X, Ubuntu
I know this is an old post but I don't find the newer one to be any better. So I wrote my own skinning guide, and its made my Ubuntu machine just like a Mac. You can see for yourself over at the Ubuntu Forums: http://ubuntuforums.org/showthread.php?t=602540