Now here's an idea I had; not all that original as the GPS module may see the light of day someday.
Anyway, the idea would be to have very small modules that would clip onto the bottom of the iPhone, matching its width and thickness; from the drawing at right, also available at Flickr. Note that some of these would require custom software that is not yet available.
The infrared module would allow control of appliances like TVs, DVD players, etc. The laser pointer could be used while controlling a slideshow via Salling Clicker, when it is ported to the iPhone. The battery module would have a pass-through connection to allow one additional module to be daisy-chained; it could also re-charge via the standard iPod connector. The GPS example here has a fold-out antenna for better reception. The small camera flash module (turned around to face the same direction as the camera) is similar in concept to others available for camera-phones. The FireWire/USB module allows the use of other cables to connect/synch/charge the iPhone. The same with the Ethernet RJ-45 connector; you could connect to a wired network where WiFi is not available or not allowed as the iPhone is not an approved device. And lastly, a memory card reader to allow transfer of photos from your camera's memory card to the iPhone, to allow easier synching as well as being able to e-mail photos while in the field.