Some stuff to get you started:

basic introduction on mapping a cube to a sphere:
http://mathproofs.blogspot.de/2005/07/mapping-cube-to-sphere.html

what you want:
http://acko.net/blog/making-worlds-1-of-spheres-and-cubes/
http://www.nullpointer.co.uk/content/?p=393

Make sure you understand how a "basic" heightfield terrain works before attempting to create a spherical terrain. Basically a spherical terrain can be created out of 6 heightfield terrains, so it's really a good idea to start with them first

[edit] Also, if you want to create MASSIVE scale spherical worlds, you should have a look at how octrees work.