Hi Pete!
Yep; we're seeing more and more single-speeds at cyclocross events here. They are indeed light and avoid the jamming possibilities of a derailleur setup. Most 'cross courses are pretty level (except for the hop-off-and-carry obstacles, of course), but when the courses have elevation changes, then some sort of gearing is helpful.
I can see having the bulk of the drivetrain weight at the rear (as with a Rohloff) would make the carry portions of the course pretty awkward, as the bike would be harder to balance on the shoulder. It would be harder to "bunny-hop" as well, I'd think.
Best,
Dan. (...who likes the idea of "spare bicycles" as one can never have too many)