Haha! Man, it's such a beautiful thing when stuff comes together. I had a lot of issues getting this to work. Again, I'm suffering from my lack of math skills with this. I guess I should quit whining and just buy some videos from the Teaching Company, but thankfully I was able to find the formulas worked out by somebody else. Some copying, pasting, and tweaking, and viola! Gravity balls! I still have a long ways to go, obviously, but I think there's already enough here for you to give this a spin and see if it's something you'd enjoy.
It's been awhile since I worked with Unity--been too distracted with Legend of Grimrock. I decided to try to get something done today, breaking apart the project into bite size chunks. I figured I might as well start tackling the heart of the game first, and spent the morning getting a basic system worked out for orbiting the planets around the sun. I'm not quite satisfied with this--for one thing, I can't seem to work out a way to make the planets rotate in sync with their orbit. I'm guessing there's some heavy math involved in something like that, so I might just fudge it. No reason (I can think of) why I can't just pick some textures that won't make much difference how they're angled. In other words, making a ringed planet like Saturn look right would probably drive me nuts, so I might just stick to rocky worlds and gas giants. At any rate, I'm fairly pleased with this part. Now I just need to make some ships that will rotate around their respective planets.