Supposedly, anything that works with a current generation iPod or Nano should work. I've already done some checking to make sure my FM transmitter (Belkin Tunecast II) will work. They've got a ton of accessories already listed on their site for the iPhone.
Exactly. I don't see why any of them wouldn't or couldn't work. The connector, as far as I know, is exactly the same, and thereforee there is no way to stop any of the accessories that work with the iPods from working with the iPhone. I think.