If you purchased an iPhone on a credit card with a price protection plan, you can call them up and see if they will honour their return policy protection with the iPhone price drop. In our case, we've personally confirmed that American Express is offering the drop. All you have to do is call their return number.
As long as you're under the 90 days that their plan covers (which is everybody right now), just call 800-297-8019 and tell them that you're inquiring about the iPhone price drop. They should know what you're talking about, since they've been getting so many calls the past two days. The service representative we spoke to was extremely nice, only asking for some details like when you purchased the phone, whether you bought it at an Apple store, and how much you paid for it.
The rep said it wasn't usually covered under their plan, but since they love their customers so much, they're going to honor it this time. And that's why we love American Express. [Amex]