Well, I searched through all the cyclocross threads and I didn't see anything on this subject.
Singlespeeds seem to be popular in cyclocross now (I don't know about the Pro or Elite ranks).
What are the advantages, besides lighter weight and simplicity (less stuff to damage)? Single speeds seem to make a whole lot more sense since, having dabbled in the sport myself, not much shifting is done anyway. I only used two gears on the whole course when I 'crossed and I never used the large chainring. Are they even legal in the upper ranks? I have found the sport so intense that there doesn't even seem to be time to shift, so why bother? I'm sure it would depend on the course, however.