I did some stuff with terrain LOD by using tessellation, but that is a DX 11 feature and not supported in WebGL yet. Basically I rendered Bezier patches that were turned into triangles on the GPU, and then used screen-space displacement mapping for the finer details. The bezier patches were designed to be seamless but still add/remove triangles as the distance grew. Unfortunately I no longer have the code, but here is an image:
