I first read about blowtorching a prime rib roast last year in Thomas Keller's Ad Hoc cookbook. The basic idea is that you 'jump-start' the Maillard reaction
with the torch, and then cook your roast through at a much lower temperature in the oven. The result is an incredibly even doneness, achieved without sacrificing the beautiful and delicious brown exterior that usually requires very high oven temperatures.
The old-school method of throwing a large roast in a 500º oven leads to well-done meat on the outer area of the roast with a small interior area of medium-rare in the center. By lowering the oven temp and giving the roast more time to cook, you can control the doneness of the entire interior of your roast. But without the high temps, you've got less-than-ideal flavor and ugly, gray color.
Converse to the old school method, Alton Brown recommends starting a roast at a low temp, and then as desired doneness is reached he recommends cranking the temp to achieve a nicely browned exterior. The results are much more even than the old crank-it-and-go method, but the blowtorch technique results in an even more even doneness.
So how do you do it? It's incredibly simple. First, preheat your oven to 275ºF. After giving your roast 15 -20 minutes to temper out of the fridge, place it on a roasting rack and evenly torch the exterior. Don't waste your time or money with a small butane torch from Williams Sonoma, just get the cheaper, larger, better propane torch from the hardware store (a big plus is that it has many other uses in your kitchen). You don't need to fully brown the roast with the torch-once it starts to turn gray it will continue to brown in the oven, even at the low temp. If you want a darker roast, torch until it turns slightly brown. Once you've evenly torched the outside, season liberally with salt and pepper, then put it in the oven. Once it hits an internal temp of 128ºF, pull it out and let it rest before slicing for perfect rare-medium-rare roast.