Our new graphics engine which will be coming out in a couple months is based on the open source Irrlicht graphics engine which is Windows, Linux, and MacOS. We're just going to support Windows at first, but in theory, versions for the other systems should be pretty easy as we're using cross platform sound and networking libraries as well.

I am torn, though, in deciding whether or not to support these other platforms even if the port is quite easy. I don't know if the extra people that it would bring in would be worth the extra QA to maintain three different versions on the client.