Unreal Nation has posted an e-mail they received from Epic's Tim Sweeney, asking about their future plans in regards to cross platform portability. Here's a quickie excerpt:

We will develop the next engine on at least two platforms simultaneously. At least PC and Playstation2 (pending arrival of a development system). I can't promise Linux, but it would be pretty easy I think. The big complication we've faced porting Unreal Tournament is that our C++ code needed lots of modifications for Gnu C++, but since that's the Playstation's compiler too, our code should be Linux-friendly all along