I've put some time in with the rear disc now, and I've got a few observations to share. My setup is a rear BB7 Road caliper, SRAM cable, full length Pit Stop housing, and Rival shifters/levers. Front brake is an Avid Shorty Ultimate, set up in the Senater from Wyoming (i.e. "wide stance") orientation - same cable, housing, and levers as the rear. The disc fork hasn't arrived yet.
I've got fairly extensive use of BB7s on my old mountain bike, and had them working really well. The basic setup is precisely the same as it was on the mountain bike, with the exception of the levers, so I know how to set them up and how they feel when they're working to their potential. I've got a few early thoughts on their operation (the road BB7 isn't as powerful as the mountain version, the Speed Dial levers on the mtb really help you tune the brake to how you want it, road levers don't offer the power or adjustability that Speed Dial levers do, and that still and all they work pretty well), but what I want to focus on here is the logistics of using discs for cyclocross.
In short, it's a pain in the acorns.
If your mountain bike isn't a 29er, or if you don't have a mountain bike, disc cross wheels will be totally unique to your cross bike. It seems that the world is leaning toward using 135mm spacing for cross discs, but if your disc-equipped cross bike is spec'd for 130mm spacing, you won't be able to use your mtb wheels on your cross bike. While waiting for some new hubs from Novatec, I am using my 29er wheel as a cross wheel. This means my 29er is angrily hanging without a rear wheel. Many people don't have a bunch of 29er wheels hanging around like they have extra road wheels.
Even if you do have extra 29er wheels hanging about, the tires that are on them are too fat for your cross bike. This means that to switch back and forth from mountain bike to cross bike use, you are either going to run a REALLY skinny tire on your mountain bike, or have to switch tires whenever you want to go back and forth. The same is somewhat true of swapping between road and cross wheels, but not really - whether for warmups at the race or riding on the trainer at home or just getting a bunch of miles in on your cross bike in order to acclimate to it, a road tire on a cross bike is very useful. So if you only own two sets of road wheels and have a cross bike and a road bike, you'll have to switch tires to turn your "road" wheels into pit wheels, but your road wheels with road tires have high marginal utility on a cross bike. 29er wheels with 29er tires have zero marginal utility on a cross bike.
A mountain bike cassette is also useless on a cross bike. My cross bike's rear derailleur can handle up to 28 teeth. My mountain bike cassette is an 11x36. It's a full on "change everything" when you want to swap wheels. My preferred road cassette is either 11x23 or x25. For cross I'm starting with an 11x26. I wouldn't have wanted a max 23 at Apple Cross with all that mud, but we're at the margins. During cross season I could EASILY live with 11x26 on all my road wheels.
The 135 spacing also comes into play in weird ways. You'll have to adjust your trainer. That box full of skewers you've accumulated? Useless. That trainer skewer that you use so your bike doesn't slide out of the trainer is useless.
So in the early returns, during which all of this "inconvenience of an entirely different format" stuff became evident without my really having gotten a feel for whether the performance of discs is profoundly better, I'm not a prophet for discs. This is a really important decision for us, since unlike the big brands who can't afford to not have a disc cross bike in their lineup, and who can just shrug it off if the format falls flat on its face, we can't. We have to make the right choice, and that means evaluating it from all angles. Those angles will inevitably include the prevailing tide - it's easier to have the same format of equipment that most people are using. So we have to pay close attention.
As a sidenote, I rode the Cabin John Trail last night and came away with a few related impressions. First, it's pretty amazing how much stuff you can still clear with a cross bike. With few exceptions (one creek crossing, one root- and rock-littered ramp that became a runup rather than a ride up), I rode pretty much everything. On the other hand, it became fully evident how much difference 2.1" tires, 100mm of fork travel, hydraulic discs and flat bars with one finger brake levers make. You can FLY down stuff, fully knowing that the stuff you bash into doesn't make that big a difference, and confident that if you seem something that's just too burly, you can scream to a stop as needed. The time I got a little bit too comfortable and really let the dog off the chain, I was soon rewarded with a pretty serious smack into a root ledge, when I became convinced that I'd broken my bars, fork, and head tube (and flatted and cracked my rim). Fortunately none of the above happened, but sheesh. Five minutes later I got a little loose with it again and twanged my left shift lever to an entirely new position.
Maybe these impressions will all change but for now, if the choice were mine, it's cantis.