Rene Ritchie, TiPB, posted an interesting article today with an answer to something I was trying to find out myself. He connected his iPad Camera Kit to his iPhone 4 to see if it would work. It did NOT.
When he plugged it in he got the "This accessory is not supported by iPhone" message.
Now this is a little odd because the iPad runs 3.2 while the iPhone 4 runs iOS4 with a lot of the same specs(if not better). You would assume Apple to allow this for us to view our pictures on the beautiful Retina display:iphone4vert:. But nope, not yet at least.