Heat grill over medium heat and spray rack with non-stick cooking spray.
Grill corn, turning frequently (so they don't burn), until the corn is slightly browned and heated through. (about 8-10 minutes)
Meanwhile combine the mayo and sour cream in a small bowl and whisk until smooth.
In another small bowl combine the chili powder, salt and pepper.
Slather the hot grilled corn with mayo/sour cream mixture (I use a pastry brush to brush it on), then sprinkle with the chili powder mixture. Coat in the parmesan cheese, and sprinkle with the chopped cilantro.
Serve with a lime wedge (use the lime to squirt lime juice all over the cobs).