Trim fat from lamb roasts. Place lamb rib roasts in a large glass baking dish. In a small bowl combine garlic, ginger, curry powder, cumin, and salt. Wearing disposable or plastic gloves to avoid staining hands, rub spice mixture evenly over meat. Spread roasts with yogurt to coat. Cover roasts and refrigerate for 4 to 6 hours or overnight.
Insert a meat thermometer into the thickest part of the roast, making sure the end of thermometer does not touch bone. In a covered grill arrange medium coals around a drip pan. Test for medium-low heat above the pan. Place roasts, bone side down, on grill rack over drip pan. Cover and grill the meat for 45 to 60 minutes or until thermometer registers 155 degree F for medium doneness. (Or place bone side down in a foil-lined shallow roasting pan. Roast, uncovered, in a 325 degree F oven for 1 to 1 1/4 hours or until thermometer registers 155 degree F for medium doneness.)
Cover meat and let stand about 10 minutes before serving. Makes 6 to 8 servings.