As promised, by our beloved Steve Jobs you can now order a free case for your iPhone 4 but before that you have to follow the given steps.
First Download the iPhone Case Program app from the App Store by clicking here. When the app is installed launch and login using the iTunes account. You will be offered to choose a case from the given list. Now after you have done wait patiently for minimum 3 weeks.
The other very feasible option is going to ebay and get one good looking case as it is much better than waiting for 3 weeks.
Here is the list of iPhone 4 case reviews on youtube, watch them, compare with the ones offered by Apple and make your own decision.