More details about my findings when trying to create spherical reflection maps…
This is a scene from the iPhone-game I’m working on:

I placed a sphere in the center of the scene, applied a 100% reflective material (no specular or other stuff) and tried Render > Bake to Render Outputs…

Well… Not all that great. Turns out the position of the camera is quite important here too. The trick is to place the camera inside the sphere:

Voilà – perfect result.

PS: Well, a couple of other things needed to be set up too… Render/Global Illumination: Ambient Intensity to 1.0, Ambient Color to pure white, Final Color Output: Gamma to 1.0…

Well, just see for yourself. Here’s a file containing the setup for both cubemap rendering and spherical map baking: cubemapBakingSetupEmpty.lxo

Edit: You can now set your camera’s projection type to spherical in modo! Even easier!