Planet Rendering

I think the most logical awnser would be "level of detail".
I dont get what "How do you deal with gravity the isn't always in the negative Y direction?" has to do with the rendering of the world though.

just use a vector pointing towards the center of the planet as the force vector for gravity.

If I'd got it right from his website Ysaneya uses double percision coordinates.
Sean O'neil has got an interesting LOD algorithm which works (for me) very similar to the one Ysaneya has. See http://sponeil.org (sources included).

I'm using an other methode. I'm using space partioning into segments having their own coordinate system. The problem with the Y direction is solved very elegant.
And I'm rendering into screen layers, too. Check Koci's post http://www.gamedev.net/community/forums/topic.asp?topic_id=422162. He has got a similar idea of these layer drawing and he gives much useful hints.

Heighmaps:
I found the trick of ysaneya good to map a cube to a sphere: Every side of the cube can contain a squared heighmap which you can easily refine for each LOD step. I'm using perlin and Diamond square midpoint displacement.

Another good point to start might be the planet renderer of Jochen Winzen; it also includes all source code: http://www.vterrain.org/Packages/IUE/