I received my $100 credit at the Apple Store today to compensate me for purchasing the iPhone when the price was $599. To get the credit, I did the following:

1. I visited the Apple Web Site iPhone Store Credit page at: http://www.apple.com/iphone/storecredit/, and clicked the Get Started button.
2. I entered my 10-digit phone number and my iPhone's serial number (engraved in small type on the back of my iPhone, directly above the letters "8GB") into the web form, and clicked the Continue button.
3. I received a free text message from AT&T containing an access code. I put that into the next form that appears in the same browser window and hit the accept button on that form. (I can't remember how that button was labeled.)

After I hit the accept button, I was shown a facsimile of an Apple gift card with a 16-digit Credit Number and an 8-digit PIN, with corresponding bar codes. I can print this card and take it to the Apple Store, or copy down the numbers for use in person or on-line.

There is currently a small graphic on the Apple Home Page that says "Early iPhone owners: $100 Apple Store credit available. Click here." The link that this image points to gets you to the same place, so you can follow the procedure outlined above.