HBO got thrown a rather substantial (and technical) bit of information yesterday by Jaime Griesemer, Senior Designer on the Halo team. Lloyd Wright, a visitor to HBO's forum, asked the question:

Does Halo use tiled regular grids (height maps) for the terrain, or an irregular grid with some clever Level Of Detail algorithm [?]

HBO decided to ask Bungie directly, and Jaime directed them to this article at Gamasutra. You don't need a degree in rocket science to make your way through it, but it's up there. Good catch, guys!