I hate how many cases ruin the aesthetics of the phone, or even worse, get in the way of my fingers accessing the touch screen (like many leather cases).
However, I need something there to protect the phone from drops. Drops happen, almost to everyone. Most likely, you WILL drop your iPhone one day. It may survive, it may crack. Do you want to take the chance? Plus the 3G has a pretty slippery surface, especially once it accumulates some skin oils.
I tried one of those rubberized gel covers from AT&T for a while. Worked okay but felt cheap and trapped dirt.
I just got a Seidio Innocase II Surface in black. Love it. The case is hard, thin, fits tightly, looks good, and has a rubberized surface.
I do not use a screen protector. IMO the glass is sufficiently resistant to scratches, as long as you don't put anything else in the same pocket with the phone. I just want to protect it from drops primarily. The phone costs way too much at full retail for me to take the chance.