I was thinking about the lack of Bluetooth keyboards and came across a decent reason for it's absence.
Right now you hold the iPhone to use it and touch the screen to navigate.
With a Bluetooth keyboard you have to put down the iPhone - presumably into some kind of holder/dock to make looking at the screen comfortable. So this require is already ineligent and means you don't need Bluetooth at all; a hard dock on the keyboard keeps the phone screen at a reasonable angle.
Now we have navigation. Without a mouse how are you to navigate or even launch apps? If you've just docked your keyboard or hung it in your homemade paper-thingy, are you really going to pick it up repeatedly? You would need some form of cursor to support a mouse or trackball.
Now we have navigation.