i will probably get a slip on case. I personally hate the screen protector things. Every time i see someone use one I scratch my head and ask how it doesn't drive them nuts. They inevitably have either bubbles or misalignment.. i can always notice it. The screens are tough on these devices. and as someone else said, the screen on the 3GS is amazing. I do use an appropriate cloth and wipe it off before viewing video, but with the 3GS that is almost unnecessary.
The iPhone, for me, is such a nice design, I really prefer to use it without some cover on it. I do like to protect it, and much more so for iPad, when i throw it in a bag or something.... thats where a slip on case is perfect.