I just came across an interesting planet-surface generator, called the "Infinite Universe Engine" (IUE), made by German programmer Jochen Winzen.
Although hardly "infinite", it is able to render 3D-planetary terrains in real-time on many of the worlds in the Sol system (like Venus, Mars, Phobos and Deimos, and of course, Earth), so you can travel from space to topographic terra firma without any loading time in between, and it also features minor noise-elements such as trees and plants on Earth for the minor details.
Anyway, I thought this might be of interest to the future of Celestia - just think if you could find a way to combine these two products, for example, or use the approunch of one in the other (actually, the creator of the IUE mentions Celestia in his papers as well).
The engine is free, and can be downloaded from here:
http://www.vterrain.org/Packages/IUE/index.html
Although he says the minimum requirement for the program is about 1 GHz, it runs fine on my 733 MHz (and I also heard of one which got it to run stable on a 500 MHz!), with just minor loading lags when I close in on a planet from space.
Most importantly: Have fun!
!