Unless Apple update/allow input devices as another bluetooth profile in addition to headset, I would not think that would be possible. I have the infrared virtual keyboard /bluetooth that I used with my Palm devices, so I can only hope - but it required a driver in the Palm OS to be installed, so may be challenging to do this on the iPhone.
I would think something using the Apple i-connect slot may be possible, but I would not buy something like the fold out keyboard, I would probably like the virtual keyboard or a roll-up rubber keyboard.