We recently just finished up with the first commercial for CCP's Dust 514. Everything in this spot is made of sand and particles. There is literally no geometry being rendered. After trying a few options, we went with some proprietary MPC tools for caching some custom particles and nParticles in Maya. These were then rendered in Renderman since it could handle the high number of particles with self-shadowing.
Adding up all 7 shots, the total particle count comes to just over 570 million.