When I had my 3GS, I had a "
Power Support Air Jacket" on it. Covered the horrible scratch-prone plastic back, left the front free.
Mrs D has an "
Incase Slider" for her 3G. Covers the back & wraps around the sides. The bottom part can be removed so the phone can be placed in a dock.
I never bothered with a screen protector on my 3GS, & it was still scratch free after a year.
I did use a "
Power Support anti-glare" screen protector on my 3G, which was quite good and didn't affect the screen in any negative way. Like all screen protectors, though, it was an absolute PITA to apply without bubbles or dust.