I'm not too rough on my iDevices, but if you are, consider the Griffin Survivor iPhone case. At US$49.99 it costs more that most other cases, but it will likely protect your phone in dire circumstances that would compromise others.
Griffin claims the Survivor meets or exceeds US and UK military specs. In fact, it's really two cases. An inner plastic case surrounds your phone, hugging it closely. You can open the case with a handy slot that accepts the edge of a coin.
Then, you slip the inner case into a rubberized cocoon that adds thickness and protection. There is also a shield that protects the display. Every port is covered, so you should get good immunity to dust and moisture. The home button and volume controls operate with the phone in the case. All the other ports have a rubber flip-up cover. The front facing camera is exposed for using FaceTime, but is behind the glass shield. A removable belt clip allows you to carry the phone either horizontally or vertically.
Throwing caution to the wind, I dropped the phone a few times on a hard surface from 6 feet with no ill effects. I wasn't willing to risk anything more aggressive, but I get the point that the case is solid. You can view some torture tests of the case on the Griffin website that are truly impressive.
There are a couple of things to think about. First, this is not a case for casual use. The rear facing camera port is covered, and the rubber plug has to be removed to take a picture. Same for charging the phone. That port is covered, and the belt clip has to be removed to charge it. This is understandable, because the whole point of this kind of case is to seal the phone from danger, but casual users who want to grab a quick photo or do a quick recharge aren't going to find this case the best match for their lifestyle.
On the other hand, if you work in a moist or dusty environment, tend to drop your iPhone and want something that will give you peace of mind, the Griffin Survivor is the case for you. The case is not waterproof, but will offer excellent protection against wet environments.
The fact that the case meets or exceeds military specs is not meant to imply it is tested or approved by any government. Griffin has had the case independently tested to meet military specs for blown rain, blown dust and grit, vibration, shock, temperature and humidity. As I said, it's a solid product.
The case fits both the Verizon and AT&T iPhone 4 models, and is available in olive, white, pink and black. There is also a version for the iPod touch.