I know many of us want this feature and is one of the reasons we want the source code, but I'd just like to state a suggestion, think I saw a small trailer in some obscure now dead or near dead mod somewhere + my own ideas. Just saying HOW it can be done, suggesting it is tested.
You can search round moddb for the trailer, its from another server so though dead if I post it here I'd likely bet banzored. :rtfm:
Anyway, the technique (or what appeared to be the technique from what I saw).
The base. Made something like the floor thing of that nomad level where we had to shoot the generators while flying in the trench (ahh... fun running in circles blasting stuff... makes me want to play SP again the 4th time this year). Just that arranged the way we want it to be. The sky will be a little hard to do as you can fly as high as you want, then again can be countered by another floor thing just made to look like clouds. Buildings can be done as well pretty easily I believe. As for falling off the sides, just make formless ocean around it. Or sandy desert. Whatever. Same as in space, it is possible to head off system. Unless you can block it with another giant invisible wall.
How to do this? Replace all dock rings with jumpgates, that jump to this 'planet' system. Where do we land then? A 'base' on the planet. Same deal, approach and dock.
So, it is not about 'can you do it'. Its about weather you want to or not. I'd like it very much and would try help with terraining and stuff and making things look good (about the only thing I can do from experience as a warcraft3 mapper).
Gravity. Well thats a tough one. Lets just assume ship engines can overcome gravity. As seen on planet environments when we land on existing planets, they can. And getting out of your ship is beyond me, sorry. No comment on that.
Sorry if the idea ain't too popular, but I've been led to believe explorable planets was one of the main reasons why we want the source code anyway.