Ex has the right of it. The issue is not raw stopping power as such; even on mountain bikes, canti and v-brakes have more than sufficient raw power (under dry conditions, of course). The major advantage of disc brakes, and hydraulic disc brakes in particular, is much superior braking modulation and progression. Performance in wet weather is another advantage, but it need not be considered in an assessment of how disc brakes are superior to caliper rim brakes. Having much more braking power available doesn't mean stopping faster, it means having a great deal more control over the degree of braking force that one is applying.

I think the worry that the increased power of disc brakes will lead to more crashes from riders slamming on the brakes is just silly. Yes, riders will need to adjust, but it seems as though a lot of Cat 4s aren't adjusted to the relatively weak caliper brakes that they already have, so I don't really see it as making things worse. Crashes aren't caused by powerful brakes, they're caused by bad riders.

I think that the rule change does make a development path toward a lightweight road disc brake considerably more likely, if not inevitable; the question is whether it will see widespread adoption. As WR points out, there are trade-offs to be made. There are practical obstacles that are bigger than the safety issues. For example, a disc brake is really only necessary on the front wheel; do manufacturers go the simple route and do front and rear discs anyway, or do they do the more sensible thing and go disc front, caliper rear? If they do, that introduces a host of problems; I'm guessing the former is what most do. Or, will manufacturers be willing to make and sell high-end control levers in both cable and hydraulic formats during the inevitable transition? Or will they throw their eggs in the disc brake basket (a la Shimano SIS "trickle down" from DA starting in 1984)? The issue is that new hubs, wheels, rims and forks are all required. It's actually a much bigger deal than SIS was in the 1980's. Shimano makes those parts, but they don't have the plurality they did in the shifting components market, and these are much pricier parts.

In any case, consider me a very interested observer. The 2011-2012 'cross season will probably be very interesting, so keep an eye on that if you want to start thinking about what a crit will look like with disc brakes.