Coding the physics wouldn't be too hard, but editing "spherical levels" would be tricky at best. Each item would have to be rotated according to its placement in the world. And, unless the level was very big (i.e. Earth size) the visible curve would be very noticeable (cool looking, but probably not what you want).

The good news it, you don't need to make your world spherical in order for it to behave that way. If your world is large enough you can split it into sections (either levels or a "treadmill"of map entities). Each section can be flat. When the player exits one part of your world they will enter another part (i.e. using level load). You can link these levels into a loop so, when the player leaves the left side of level 'A' the will enter on the right side of level 'B', when they leave the left side of level 'B' they enter back on the right side of level 'A'.