VIPM == View Independant Progressive Meshes
A type of runtime triangulated irregular network. Another is VDPM, view dependant progressive meshes.
In VIPM, the collapse order is fixed. The only thing the observer’s position does is choose how many tris to expand/collapse to.
In VDPM, the collapse order is varied. The order of collapse depends on where the view is, so tris further away from the camera get collapsed before tris near the camera.
The demo Thr33d mentioned appears to use some sort of VDPM as its LOD. Quad-trees and ROAM use regular grids.