Hey, I got volumetric ray casting to work in Blender Game Engine. The shader is fully done in GLSL, including front&back face calculation in single pass and 2D texture conversion to texture3D, so it should be reeaally easy to implement in every OpenGL supporting engine.
The blend. file is here:
glsl fragment shader: (might crash on ATI/AMD gpu`s. debugging in progress..)
screenshots taken in BGE (click on the thumbnails to see them full-sized) :
different opacity values
simple sample dithering