Radiance works on Linux, too and it is allready free for non-comercial use. When it will become Open Source, it should be free for any non-comercial and comercial purpose. I’m sure you know POV? Radiance is working in a very similar way: you have to create your scene in an ASCII file. A more convenient way to create a scene is to use an AutoCAD version 10 .DFX file and translate it then to the Radiance file format. There are some more translators form/to other file formats available. But most of them are CAD formats.
Due to the fact that this software will become Open Source and a lot of Blender users would like to have Raytracing (including myself) this program should merge with Blenders code if possible. Or at least parts of it. I don’t know much about software development but it would be nice to integrate this powerfull render engine in Blenders source code. Radiance is, like Blender, written in C. If this would be possible, we would have a very nice modeller and a very good render engine. The other way is to integrate a good(!!) export feature in Blender that supports Radiance (or another good raytracer like VirtuaLight, BMRT …).
IF Blender will become Open Source, all this would be possible. And I hope it WILL become Open Source.