The problem with a realistic simulation of planetary rings is that it essentially an n-body problem with millions of bodies. The n-body problem does not have general closed-form solutions for n>2, and even the fastest general simulations run in time at best O(n log(n))[1]. I did a research paper about the n-body problem, once upon a time.
I suggest that attempting to simulate planetary rings should not be something that Planetside should worry about--at all. The vast majority of applications for a program like Terragen require not realism, but plausibility--it has to look "good enough." Very few movie-goers, for example, will really care if planetary rings are not physically realistic, as long as they are visually plausible. Sardine has already demonstrated that it is possible to create visually plausible simple planetary rings with the tools we currently have, without excessive difficulty.
What of more complicated planetary rings, though? For example, what if you want to do an animation of a planetary ring? (I assume that planetary rings rotate... I'm not an expert on them specifically, though). Terragen does not currently allow rotating populations as a whole--if you could do that, while preserving the specific instancing (which makes it even more complicated), then it would be an acceptable way to make animations of planetary rings on sufficiently small scales (or from a great distance), and over sufficiently short time frames. And for any yet more complicated or realistic requirements for planetary rings, I would suggest that Planetside let somebody else write a plug-in for that. This sort of third-party extension--both free and commercial plug-ins--of 3D programs has a long and productive history.
Maybe my long history with Terragen--it's been 8 or 9 years since I first stumbled across it, now--has given me rose-tinted glasses, but I honestly can't say I find any fault with the way Planetside has gone about developing it, or marketing it, in the past[2], nor the direction it appears to be headed in the future. Of course, like everybody else I would like a program that does everything I could possibly wish for the low, low price of $4.95; but Terragen, for $99, $199, $299, or whatever it might end up costing, is an exceptionally capable program, and for a company that has two main developers (Matt and somewhat more recently Jo) Planetside has done some truly exceptional work over the years I've been around.
[1] Due to the nature of a planetary ring system, I expect it could be roughly simulated considerably more efficiently with a dedicated algorithm; perhaps even O(n). However, for a ring with millions of objects in it, even that could take a good long time to simulate.
[2] Including Matt's trip across the ocean to DD and the ensuing delay, the delay in the Tech Preview's release which caused so many ill feelings, and so on. Obviously, it wasn't always the best possible PR, but stuff happens. Things go wrong. Perhaps I am patient and/or forgiving due to my long history of making mistakes, missing deadlines, and so on...
P.S. Will, it's
spiel--from German, literally meaning game. It's taken on a somewhat different meaning in English, though.