I had a roll of this black tape that people us to cover up chrome trim on BMW's to give it a sportier look.  We call it shadowline.  It's like a flat black and very thin but strong and stretches a bit.  I used some to go over my iPhone 3G and it works great for avoiding scratches!  I did just a demo so it's not cut too well but if I can make some sort of template it would look awesome!  It's not near as thick as the skins people sell and it covers the whole back.  I could probably stretch it to go up over the chrome for a completely mat black look.  As much as I love Apple it's neat having a stealth iPhone without the shiny black back and big chrome logo.  Plus no scratches!  It does become harder to take off over time (like the skins) but it looks great.  I will take pics when I get it perfect.