I wasn't using a screen protector on my iPhone 3GS for a number of months. Then I ordered one for 'just in case', that I ended up procrastinating putting on, and of course I ended up accidentally dropping my iPhone in the Post Office parking lot while mailing packages.

I was actually writing addresses on the packages and set the phone down on my lap for a second, then I while thinking about the packages I stood up and the phone dropped face first into the ground. no major scratches on the screens, just some tiny ones that I find annoying. Wish I would have put the screen protector on.
I purchased this case that came with the 'mirrored' screen protector which looks great.
http://store.tipb.com/case-mate-bare...4A123A5564.htm