I know we have talked about parallelizing the main engine in the past, and the general response was that it was possible, but it would require so much headache inducing work to ensure that clients still work deterministically for network sync purposes that it probably isn't worth it compared to fixing up bugs, balance issues, and developing new game. That I get.
However, what about parallelizing the graphic rendering? Aren't there some good frameworks for that? Are there good frameworks for abstracting this so it isn't very painful (just say "draw" this "batch" of stuff, and the framework or the video driver or the video card would try to split it up if it can), and take care of how video cards and drivers expect it for you?
Plus, as it is a graphic thing, you don't have to worry about network sync. The exact pixels of the "pretty pictures" that on the screen don't influence what the engine thinks is happening.