I recently decided to dedicate my old iPod nano as my "car-tunes" device and since the battery is getting weak I wanted to ensure consistent power for long trips. I bought 2 USB adapters for the cigarette lighter from overstock.com. They are about $3.50 each and decided to convert a blank switchplate on my Honda's dash into a USB power port.
There is nothing revolutionary here, just wanted to add my variation on the theme. I wanted it to look as OEM as possible. I think it turned out real clean.
Dremel with shaping bits
USB 12v power adaptor
hot glue gun
about 1.5 hours total time
Step 1: The USB Power Adapter
It didn't take long to find inexpensive 12v USB power adapters. I found these at overstock.com and they are from eforcity (not a promotion). I was looking for something with a small form-factor and that appeared easy to disassemble. I bought two ($3.50 ea) in case I messed one up, but didn't. I put the second one in the "power port" in my wife's car for the kids to use.