Despite not adding much bulk to the iPhone, Griffin's Motif is one of the more protective cases available through the iPhone 4 Case Program, and the Motif's appearance and feel are unique.
The Fitted Case provides good protection from bumps and drops, as well as a unique style. As long as you don't need to use larger third-party cables and plugs, or use a dock-cradle accessory, the Fitted Case is a solid choice.
Belkin's Shield Micra offers moderate protection without adding much bulk and without hiding the iPhone's design.
The Reveal Etch combines the svelte protection of a plastic shell with a bit of the cushiony edge protection of a silicone "skin" case, throwing in a carbon-fiber-style back panel for good measure.
The Snap Case doesn't add much bulk or hide the iPhone's good looks. It's also an appealing option if you plan to use your iPhone with third-party accessories, and the included stand is a nice bonus. But the case is quite slippery and doesn't offer a lot of protection.
If you're a fan of "silicone skin" cases, this is the closest you'll get from the offerings in Apple's free-case program. It's also a good case if you plan to use third-party accessories, thanks to large openings for plugs and connections and an easy-to-remove design. But it also suffers from the drawbacks of silicone.
The PixelSkin HD provides decent (non-screen) protection without adding much bulk to your iPhone.
The iPhone 4 Bumper is an edges-only case that provides minimal protection, but it alleviates the "bridged antenna" issue while letting you see as much of the iPhone 4 as possible.