I have the Otterbox Defender case and it has been suggested that the best option is to pop out the included screen membrane. This way you get the protection, but you don't lose the beauty of the screen. Since it comes with a holster, the screen will be protected when not in use.
And to respond to the I never had a screen protector people: stop. Just becuase you were lucky does not mean that everyone else will be. I have had both the iPhone 3GS and the iPhone 4. Both screens WILL scratch. And no, I don't put it in my pocket with keys or coins.
Funny thing is, the iPhone 4 screen's minor (you have to look VERY hard to see it) imperfection was caused by the Otterbox Defender. A piece of dirt (?) made it inside their suppossed dust resistant case and rubbed my screen. Unfortunately, since I am soon to be a truck driver, I have no choice but to use a case that will protect the phone. Hopefully, I can find something more dust resistant than Otterbox.