Yes you will be able to run around the surface, initially in first person view and later when we have animations in place also the 3rd person view.
Modifying the terrain in-game isn't easily done - currently the terrain uses real Earth elevation data in ~76m resolution, and then the engine continues to refine the terrain using fractals. If we wanted to add a possibility to modify the terrain, we'd have to store the modifications in multiple places - for multiple levels of detail - and that means overrides for wavelet-compressed elevation data and for the fractal-computed levels. That is a rather complex issue, but we plan to deal with that later. Actually, roads already modify the terrain but they are stored as splines with attributes.
Making your own planets should be easy. The height maps can be of varying resolutions, though. You can have some central game area in better detail, and rest of the planet defined more coarsely. However, the fractal part refines everything down to the centimeter resolution so you'll be seeing the detail everywhere, only having less control over the topology of the coarsely defined parts of the planet.
Color maps - that's a different problem. The engine generates and textures the terrain according to a land type model, and you'll be able to roughly specify what land type should be where - like a dessert, savanna, forests (of what type), arctic and so on. So the color will actually specify that. More detailed control will be possible by using vector-data based edits of terrain type, for example you should be able to make a corn field by setting up a polygon constraining the area and assigning a type. These artificial constructs may even be subjected to some degradation coefficient, so that for example the original land type could creep in with time in a fractal manner.
Although for some rocky planets without vegetation it could be possible to use the color directly to tint a generic material.
We are also currently working on COLLADA import so you should be able to import 3D objects to be placed in game, after fulfilling certain requirements. In fact there are some very basic buildings already in there.
I expect that a demo could be available sometime in January. It should feature some drivable vehicles and a flying mode for exploring the planet, also ability to place some stock buildings. Not sure what other features get through - maybe a flyable plane, placement of roads (and airfields?) etc.
