In a small bowl, stir together the yogurt, capers, shallots, and season with salt and pepper, to taste. Let stand to allow flavors to mingle.
Cut a deep slit about 1/2-inch long in the bottom third of the shrimp, without cutting all the way through. This will allow the shrimp to butterfly open at the ends as they cook. Toss the prawns with oil and season well with salt and pepper. Grill the shrimp, turning once, until bright pink and just cooked through, 1 1/2 to 2 minutes per side.
To serve, put a dollop of yogurt sauce on a serving plate; stand the shrimp upright with their tails touching around the yogurt. These are terrific with a small salad, rice, or pasta on the side.