Wash the sweet potatoes, pierce them in several places with a fork and bake them until soft (about 45 minutes, depending on their size).
Meanwhile, prepare the dressing; combine the yogurt, mango chutney, curry powder, onion, and lime juice in a bowl. Store it in the refrigerator.
While the dressing chills, seed and chop the bell pepper, chop the celery, thinly slice the red onion, and mince the chile, if desired. Place all of the ingredients in a large bowl and set aside.
Remove the baked sweet potatoes from the oven, and when they are cool enough to handle, peel and cube them. Add them to the bowl and gently toss with the other ingredients. Carefully stir in the dressing.