hey thom - yeah, disc brakes are an option. you can find some older bikes, from companies like lemond, salsa and redline that have disc brakes. some new bikes (redline) have disc brake fittings. and because the UCI (and therefore USA Cycling for our national championships) have allowed disc brakes, they're coming back. the tricky thing is that it's always cable actuated - but folks are working on hydraulic options as we speak. you may see an adapter to make a cable-pull brake lever to actuate a hydraulic brake.
we agree they have promise - especially for the poor but fast upstart guy who wants to battle the pros in a muddy race but can't afford a high-end pit bike. they're great for adverse conditions (mud snow ice rain), and they have the potential to really lighten up a rim too. and on the road, they'll prevent the rim from overheating and blowing off a tire.
the biggest challenge right now is just to find a bike with them!
You can run a road brake cable to a disc brake with no issues, Avid makes brakes designed for use with road levers. It might be tricky to find a rear hub with road spacing (130 mm) and a disc mount, but maybe it won't be. I haven't looked for one.
I have steel disc brake-equipped single speed. I have Avid BB7's on it which are the most common of any road discs out there although Tectro does some too that are supposedly lighter. I love disc brakes...I have had them on every mountain bike I have owned for more than ten years and would never go back to anything else. However, I have come to realize that cyclocross is not mountain biking despite sharing a lot of common territory. I do believe that because the UCI now allows disc brakes you will see some companies step up and develop something that truly rivals the low weight of cantis. Hydraulics? Hello? The BB7's are powerful and reliable, but they are heavy, probably adding up to an additional 1.5-2 lbs all things considered. How powerful do one's brakes need to be is the real question? I plan to build a geared lightweight rig for next season with cantis. I like the discs, but just do not think they are really necessary after racing with them this year.
I am consistently on the fence about this situation. My background as a mountain bike racer where I have hydraulics on all of my bikes leads me to want disc brakes on my cx bikes. The lack of service is great (Avid Juicy 7's that have not been bled or needed it in 3-4 seasons) but at the same time for cross I'm not sure if it is going to be an advantage unless courses become consistently more like Asper-Gavere where there is a long fast slick descent.
Will courses become more technical as disc brakes become more prevalent or is it really not going to take over all that quickly? It is going to be interesting to watch the developments.