everyone should check this out: http://osx.beyondunreal.com/changer/

it changes the background of the main UT window (which I have dubbed my "UDesktop")
I am getting ready to work on a dFb background for it..i have also converted the ChaosUT background to work with osX...you can find screenshots here: http://p455w0rd.is-a-geek.net/ut/images