Three years ago I was flying to Canada, ORD-SLC-Calgary. I had my driver's license, but they wouldn't let me on the plane without "proof of citizenship" (i.e. passport or voter card.) I'd driven to Canada many times before without any such proof, but I guess the flying requirements are greater. Anyway, they confirmed me for a flight the next day. I've always wondered if someone could do this on purpose to get around a Saturday night stay.