I have heard "Mmm-Hmm" as an alternative to "you're welcome," but the traditional "you're welcome" is usually the rule here in Vermont, anyway. A friend visiting from NYC last weekend was taken aback when the drive-thru attendant at a fast-food place said "you're welcome" as we pulled away. He nearly fainted when I told him that the people who work at the Department of Motor Vehicles are courteous and helpful.