I have been spending the last week writing my own implementation of an LOD system for a planet. It is inspired by Spherical Clipmaps, but it is not an actual implementation of them.
As I said, I began writing it last week (exactly a week ago today) and I finally have it all up and running (using real-time 3D Perlin Noise in the Vertex Shader and texture blending, no less!). Unfortunately, I have it running too smoothly, if you can believe that. I have the vertices being updated and centered under the camera every frame. This leads to the terrain looking similar to flowing water as you fly around the planet. As long as the camera is stationary, everything looks great. You can spin the camera around and look at all of the terrain around you. As soon as you start moving though, the "water" effect is very apparent.
I first tried to fix it by having the terrain position updated only once a second, but that looked terrible. So, I ended up saving the old camera position and then calculating the angle between the current camera position and the old position each frame. If the angle becomes greater than a threshold (I used something like Pi/64, yes that's sixty-four!) then I update the terrain position. This looks alot better than the time-based updating and it removes the "water" effect. Unfortunately, it also makes the terrain very "poppy". For example, if you see a mountain in the distance and you start to fly toward it, you would suddenly see more detail "pop" in as you got closer.
I think what I have now is pretty decent, but I want to have very smooth terrain, with no popping and no water effect. So, I'm kind of back to the drawing board trying to think of a system that fixes both of those problems. Not everything I have now is throw-away code though. I think I should be able to carry over alot from this project to the updated LOD system.