This deal has ended.
Buy.com has the Kensington Pico FM Transmitter for the iPod Nano for only $16.99 after a $20 mail-in rebate valid through 9/24. I think this is one of the better FM transmitters (many are junk). It works with the newest Nano and is so incorporated into the iPod that you select the FM station from the iPod's LCD display. Shipping is free. No sales tax except to CA, MA, MD, TN.