Spent quite some time working on something like that. (With a rendering backend that can only be described as insanely brilliant. Sadly, that was not my doing ;)
Incredibly dense storage for incredibly large areas (10km^3), the ability to generate arbitrary levels of detail for terrain from the source data on the fly, and the ability to "patch" any level of detail with artist-supplied data.
It's a bummer our studio got closed :(