I've found the virtual terrain project to be a useful resource. I figure some people here may already be familiar with it but I thought I'd drop the link just in case:
http://www.vterrain.org/
The LOD page has some interesting papers:
http://www.vterrain.org/LOD/Papers/index.html
In particular one that covers CLOD rendering very large "out-of-core" heightmaps. By carefully arranging the vertices and mmap'ing the file you can get the OS to do all the hard work for you (haven't actually tried it myself though).
http://www.gvu.gatech.edu/people/peter. ... tion2001a/