Version: 2.70a official (nix 64)
How to reproduce:
- Open attached file with simple Cycles particle system, bake if necessary, and inspect settings
- Render a frame with particles in it. Notice that the fast-moving particles are motion blurred as expected.
- Change display settings of particle system to "point", "circle", or anything but "rendered".
- Notice that, when rendered again, the particles are not motion blurred.
This is a bug if ever I saw one. The display draw type of the particle system should have no bearing on the motion blur of the particles when rendered.