The current particle system is homemade and would theoretically allow rotating. However we're hoping to use the DirectX particle system in a future version. DirectX does not allow particle rotation. It is unusable at the moment because it's too slow, but it will be faster with DX10. Theoretically it could be a lot faster than the current system that was developed for A5.

Due to compatibility with that future system we won't implement particle rotation. If you need that, just use sprites. They aren't much slower than rotated particles would be.