From the Gamespot interview:
"we are currently using about 30 percent of the SPUs' capabilities--with the SPUs doing lots of heavy lifting for us on rendering, visibility, particle systems, skinning, animation blending, and so on...this with scores of pedestrians, cars, fires, etc., all going on. And the best part? We've not made any significant attempts to even optimize the SPU code. I think it's reasonable to guess we could put 10 times as much stuff on the SPUs and still make our frame budgets. It's really pretty amazing."