My mother in law uses this technique and it works well. She picked it up at a cooking class that she took. One thing to keep in mind is that you need to have a a well insulated oven since you are relying on the stored heat to work on that roast for a good 2 hours. Lower quality ovens may not insulate as well and you may come up short on the internal temp. I'd keep an electronic thermometer in the center of the roast as an insurance policy if I used this method. That way I could give it another shot of heat if I needed to get the IT up.
Oh, and this method can smoke the kitchen up pretty well so keep one eye on the smoke alarm and be ready to pull the batteries!
I second MDBOATBUM on the food safety issue... as long as you hit the outside hard with heat the inside should be safe. You'll see some guys who sterilize their probe thermometers for low target internal temperature roasts in order to avoid introducing bacteria in the center of the roast where the bacteria kill temp (~140) won't be acheived.