Today Apple officially launched the iPhone 4 case program (and it literally is a program you need to use!), to order your free case for the iPhone 4. This case is offered to those with reception problems (aka - all iPhone 4s in low signal areas).
You must first go to the App store and download the iPhone 4 Case Program app. Once you have installed it, login with your Apple ID and then select from one of several iPhone cases.
They had offerings from Speck, Belkin, Griffin, and InCase. Also offered is an Apple brand Black iPhone Bumper. Speaking of Bumpers, if you had previously purchased one off of the online Apple Store, you will automatically receive a refund, and this will be your compensation for a free case. If you purcahsed it at an Apple Retail store, you can bring in your receipt for a refund. It's a little more painful if you purchased from AT&T, as you then have to mail in a rebate form.
I chose a Griffin Reveal Etch case, and hope it was a good choice. It's supposed to be a 1.6mm thin case. I assume it alleviates the antenna issues, or it wouldn't be one of those offered.
Offering a free case does help take some of the heat off of Apple, as it is the right thing to do. With that said, I don't think a case should be mandatory, and there will be those that choose to go case-free.
This free case offer is only available to those that have bought an iPhone 4 between now and September 30th.