I just finished testing a new parallel processing option for rtrace (-n). It should work on all multicore Unix (Linux, Mac OS X) machines. Download the HEAD from www.radiance-online.org and give it a whirl. Getting the -I option working was the real trick, so let me know of any differences you spot in your results if you use it.
The only major caveat is that the -o*t* (trace tree) options won't work properly, as all the output will be randomly interleaved. Not much I can do about that, I'm afraid.
Please don't ask when -n will be added to rpict. It won't. I have spent some time looking at it, and we're stuck with rpiece for the forseeable future. The good news is that rpiece works just fine on multicore machines. It's only when you open it up to a network of machines that it starts to be troublesome, and a -n option in rpict wouldn't help that situation, anyway.