I am Joe's lactic acid.
Join Date: Aug 2008
Location: Long Island, NY
Bikes: 2008 Windsor Cyclo, and some spares.
Like Little Darwin said, rear derailleurs are chosen to match the range of the cassette. But shifters don't care what type of derailleur is there, as long as they match up how many gears there are.
Brakes are a different story. Road levers and MTB levers pull different amounts of cable, and so you'll have issues with the mechanical advantage against certain brakes as opposed to others.
Generally cyclocross bikes follow road groups. The most common deviations I see are mtb rear derailleurs & cassettes. One oddball set up is the V-brake on the current Tricross sport. Usually V-brakes are used with flat bar levers, but they put them on the Tricross to get rid of the shudder problem with the old cantilevers.