I don't have my Lumia anymore, but when I did, I used this case:
Amazon.com: Diztronic Matte Back Black Flexible TPU Case for Nokia Lumia 900 (AT&T) [Diztronic Retail Packaging]: Cell Phones & Accessories
It's pretty awesome. Cheap, too. The main benefit I found was you can use car mounts with it and the "squeeze brackets" that hold the phone don't press the camera/power buttons. It also protects the chrome bezel around the camera.
The $20 Nokia case is neat, but the buttons still depress when you place the phone in a windshield dock. Not so with the Diztronic one. Makes sense with Drive, etc.