Heat a gas grill or grill pan to medium-high heat.
In a small bowl, whisk together yogurt, mustard, salt and pepper; set aside.
Brush ears of corn and scallions with olive oil. Grill scallions for about 6 minutes, turning occasionally, or until lightly browned; remove from grill and set aside.
Grill corn, turning, for 12 to 15 minutes or until browned. Remove from grill and let cool to room temperature.
Chop scallions and cut kernels from corn cobs.
Place scallions, corn and tomatoes in a large serving bowl. Pour reserved dressing over salad and toss to combine. Serve immediately.