I've had horrible experiences with plastic hard cases. Your best bet is to acquire the Incase Protective Cover, which I've used for 2 generations. The first one I used ripped slightly at one edge after 5 months. By that time, I switched to the iPhone 3GS, and reluctantly bought the same case (I figured ripping > plastic case scratching). It's still in perfect condition ever since I bought the iPhone 3GS at release. I also use the Power Support Crystal Film, which pairs up nicely with the Protective Cover.
First, the case fits perfectly snug on the iPhone. You might hear people complaining about it starting to stretch and let dirt in. This won't happen if you leave the cover on without taking it off. If you have to take it off, be careful; same with putting it on.
I've had 5 hard cases before I switched to this rubbery one. Trust me, these covers are the best choice for the iPhone