As a follow-up to the last post, I just couldn't wait to see what I could do in terms of procedural nebulae with the particle system. The initial results aren't jaw-dropping, but they do beat previous attempts by a fair amount. The particle-sim nebulae definitely have a more volumetric feel to them (not surprising, considering that they are volumetric, unlike the 2D-noise-based previous attempts).
I actually did a bit of cheating in this shot: I've composited the original screenshot with several blurred version of itself. In principle, this is easy enough to do on the GPU with textures. I just didn't want to go write that code in the simulator, since I want to keep it as a real-time application. There's still a long way to go in the procedural nebula department, but I believe this represents a step in the right direction.