The two images show some screenshots from the early stages of my terrain engine (yes, another one!) that I've been working on since Christmas Eve of 2001. I've rewritten the LOD system 3 times, first using a 9x9 vertex tile system, then an adaptive quadtree algorithm, then back to the tile system but with each tile having 17x17 vertices. The tiles can be rendered at 8 different levels of detail, using the vertex error as the factor to decide which LOD to use for that particular tile. Frustum culling is done on each tile's bounding sphere, but a quadtree is yet to be implemeneted. It gets around 130 FPS at the moment, and goes down to 100 FPS with a particle system being used (not shown in these screenshots - that's what I'm working on at the moment).
There are lots of things that are still to do - finishing the particle engine, quadtrees, level editor, models, moving sky, lighting, and more. However, I am designing the engine with dynamically alterable terrain in mind, so you'll be able to have rockets dig holes in the ground.
Cheers, and I look forward to your feedback!