If you want to be at the top of your game for 2009, then yes you're going to have to train on the road. While spending time on the mountain bike is critical to keeping your skills sharp and to stay on top of the aforementioned "short burst" efforts, you'll have a hard time effectively performing intervals completely off-road. As you can see from the chart, on the road bike, you can maintain a sustainable power effort, thus producing more kJ's and training at a higher sustained wattage. The more work you can produce over the interval effort, the more effective it is.
Side benefit of training on the road, is the ability to perform your endurance work and lay down your base, which is critical for racing at the top level. If it wasn't effective to be training on the road you wouldn't see all the top pro's out there like they are now :thumbsup: