I think you are thinking wrong way. You think about actual sphere, I\'d like to propose something like this:
Spherical gravity is easy. Just pull player into direction of planet\'s \"center\". Even so old game like Psychonauts does it (brain in main menu and Sascha\'s brain - dunno if any others, because I never went past that brain tank thing).
With planets like above, you also won\'t get any \"bending space\" issues nor you\'ll need to do something like that Py(something) does.
If you want to build something and there\'s not enough space on ground, JUST FLAT IT OUT! What you do in Minecraft when terrain doesn\'t allow you to build your big castle and you don\'t want to play on flatland? You don\'t whine and remove blocks which are in your way. Same here.
Also no \"corner cases\". Sure that you may end up walking on wall if you build house near edge of planet, but that\'s all.
And to anyone that want to say it won\'t work as it would lag as hell with dynamic terrain, guess what? There is already game with spherical gravity, dynamic terrain and doesn\'t lag at all. To add insult to injury, it doesn\'t even use blocks/voxels/whatever you want to call it, but something more complicated. What it is called? It\'s called Patterns from Linden Labs.
So I doubt it would be hard to properly implement this (both planets themselves and generation). And yes, I\'m programmer.
//edit: Here I\'ve explained ON PICTURES how gravity would work for such world. Because some people have troubles with understanding normal worlds.
From sideview, I\'ve rotated planet instead of character, because it was easier that way and you won\'t need to bend your head in strange directions.
Green arrows represent gravity force. It pulls ALWAYS, no matter of direction, towards center of planet.