A usplash theme adapted from this image:
It looks best at 1024x768 with 24-bit color.
If you are using Ubuntu, I recommend using this to set everything up: http://web.telia.com/~u88005282/sum/index.html
The .deb is located here if you're too lazy to go to the site's download page: http://web.telia.com/~u88005282/sum/archive/deb/startupmanager_1.0.7-1_all.deb
How to compile from source (I assume you extracted the archive's contents to your desktop):
sudo apt-get install build-essential libbogl-dev libusplash-dev
cd ~/Desktop/Gangsta OS X
That's it. Then all you have to do is use the start-up manager (if you're using Ubuntu) in System>Administration to add the gangstaosx.so file to your list of usplash themes and select it.
I didn't make it, but here is a login screen that imitates the OS X one very well:
It's from this guide, which I think is the best one to use to make your GNOME look exactly like OS X: http://taimila.com/osx-guide-2.php
Also, I recommend using this icon theme: