A friend of mine recently went to work for a company called Otterbox, based in Colorado.
When I finally broke down and bought an iPhone, I asked the guy at the Clarendon Apple Store what cases he recommended.
While Apple doesn't sell the Otterbox at the Apple Store yet, he had a Defender and said he swears by it. He said another coworker casually threw his iPhone across the store because he is so confident the Defender will keep it from getting scratched.
I am using the Otterbox Commuter, which is a little bit less bulky, but it works great.
If you are in the market for a case for your iPhone or Blackberry, you could do a lot worse than an Otterbox.