As for ncca championships, i'm currently at Iowa State. I just joined the collegiate team here, and i'm told that the mountain/cyclocross season is during the fall/winter, and the road season is during the spring.
I'm in iowa so we race in the North Central Conference, and there are 5 race weekends through the spring. I think that each weekend has a crit, and a road race.
there are different levels to collegiate racing...a,b,c...just like the uscf cat 1-5.
i'm not sure of the rules of nationals, but i think you either have to qualify individually for it, or each college gets to go so you'd have to qualify out of your college team.
hope that helps...you can follow kess's link to find the collegiate racing rulebooke (40 some pages), and read about the stuff...i read some of it before, but can't remember the details.
hope that helps some,
bryan