Well this might be a wrong conclusion, but having a look at the game engine source code(stubs.c i think) recently, it seems like smoke is implemented/likely to be implemented.
Perhaps someone knows more on this.
I checked out my source of Blender, but I didn’t see any real source code, just the stubs file that you mentioned. Maybe it’s just a stub for a jumping-off point in the future; smoke implementation doesn’t seem especially necessary since it can be faked with planes (or done correctly with spheres - see my signature), but I can’t really say anything since I haven’t seen the results yet.