Creating an entire planet I would like to model a fantasy world that I made and I am wondering about the technical limits. The planet is about 3 times bigger than earth and has four continents, each the size of half the USA. I am not talking about making a sphere and texturing it, I mean like Google 3D where you can zoom all the way into a single house except more detailed. Would there be any technical problems? Would the program lag from having that many vertices? Should I just model each continent separately or even each region within the continent separately?
Is this for an animation or game? 

For an animation you can use compositing to transition between several different versions of your model, each with different scales of detail. 

For a game you'll probably need to fetch higher resolution blocks of game data as needed, similarly to how Google Earth fetches higher res textures as you zoom in. 

Either way it probably won't be easy, but the result could be impressive if executed well.