Cross-post:
Raced my mountain bike....in a cyclo-cross race this weekend.
It's an interesting way to race. Kinda like a time trial. You just go all out for as long as you can and try not to blow up.
We were mostly on turf with a small amount of mud. I ran my MotoB Ti Fly with the Kenda tires that come stock (I run Panaracers on the trail). They're still a lot wider than what most of the guys ran, but at 80 psi the rolled OK. The bike is light enough that carrying it wasn't a big deal. The SID was nice over a drop-off, but none of the cyclo-cross guys had any suspension and they stayed alive, so obviously I didn't really need it.
The tires were the biggest advantage they had on me. The skinny 700c tires just roll so much easier than even a semi-slick mountain bike tire. They seemed to have pretty good grip, too. I had a pretty big advantage on them under braking and downhill. The slightly rough downhills would shake them up a little, but it was like a billiard table to me. My Avid Juicy disks killed their canti's once it came time to get on the binders. I could probably brake 3-4 bike lengths deeper than those guys. I passed a bunch of guys that way.
Overall, cyclo-cross on a mountain bike isn't the way to go, but if you just want to get out and race, then it's completely do-able. You look like a geek at the start when everyone's waiting for the signal to go. Whatever, I passed a S-load of the guys that were looking down their noses at the newbie with the mountain bike. Ultimately, I held my own, but didn't end up even close to the money. Having said that, I'll be doing it again in short order. Now I just have to find really skinny 26" tires to be a bit more competitive.