Bring 4 cups water to a boil in a large saucepan; add chicken. Reduce heat, and simmer, uncovered, 13 minutes. Add peas; cover and simmer an additional 7 minutes or until the peas are crisp-tender. Drain and coarsely chop chicken.
Combine chicken, peas, and grapes in a bowl. Combine yogurt, sour cream, curry powder, and lemon juice; stir well. Pour over chicken mixture; toss gently. Cover and chill, if desired.
Spoon 1/2 cup chicken salad into each lettuce-lined pita half.