A Flickr account, presumably by an AT&T store employee, shows some of the early iPhone accessory arrivals. Despite Apple's claims that the iPhone will be relatively resistant to scratches, it is expected that carrying cases and protectors will be one of the hot-sellers.
Of course, iPhone will also work with Bluetooth headsets, including its own Bluetooth headset, which an unconfirmed rumor points to a $129 pricetag for. iPhone also includes Apple's 30-pin dock connector, so most existing iPod accessories should work as well (car chargers, FM transmitters, etc).