I had no problems flying barefoot on a United flight from Washington Dulles to Heathrow. All the staff were friendly. And on the flight out, while I didn't board barefoot, I did potter round in the flight and had a chat with one of the stewards about a CNN documentary about living barefoot. He was all "but what about all the germs", and I'm like "it's my United flight of whatever".
United, United Airlines, flying barefoot