Heat the grill. Combine the sour cream, mayonnaise, cilantro, garlic, lime juice and chili powder in a small bowl. Grill the corn, covered, over medium heat or coals for 10 to 12 minutes or until tender and slightly charred, turning occasionally to grill all sides.
Spoon the sour cream mixture over the corn and sprinkle with the cheese.
Serve with lime wedges and additional chili powder, if desired.