I think each particle should have it's own gravity. This will be defined in the parts array. This will allow elements to gather together. This should only be when Newtonian gravity is enabled. I think this would be cool for when regular gravity is off and you put a powder or liquid, it would move to other clumps of particles. It could show how planets form and maybe orbits without a central point of gravity.
We toyed with this in the betas before gravity became official, actually. The biggest problem is that once you get more than a few hundred particles on the screen, gravity stops updating except every few seconds, which obviously has a catastrophic effect on orbits and interesting mechanics. Photons orbiting themselves is freaking awesome, but virtually everything else is boring unless confined to very specific examples.
digest: this would be an awesome console setting I think. But I wouldn't have a clue how to do that. From my earlier experimenting, if you only have a small wire and set spark with negative gravity, the effects are pretty cool ^_^