I think it depends on what type of particles you're using. If you're using MultiPoint, MultiStreak, Points, Sprites, Spheres, Numeric or Streak, then you have to hardware render them. If you're using Blobby, Tube, or Cloud, you have to software render them. If you scene uses both types, then you have to render each method and then combine them in some kind of compositing software package.
ie. hardware particles won't show up if software render them.
I hope this helps.
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