Apple announced during the press conference addressing the iPhone 4 antenna issues that not only will free bumpers or third party cases be given to iPhone 4 owners, but those that have already purchased one would be issued a refund.
Steve Jobs mentioned it would take a little while to get the program up and running. While it doesn’t appear you are able to get your hands on your free bumper or case yet, today it appears Apple has started processing refunds for bumpers already purchased by iPhone 4 users.
The process is completely automated, thus you shouldn’t have to do anything to get your refund. Also, the announcement (image above) noted that all VAT and shipping charges will also be refunded. Those who made their purchase with a credit card should receive their refund within 3-5 business days.