I seriously doubt that you're going to see the iPhone on Orange, or any other network apart from O2 for a long time yet. Orange reps were saying that the iPhone MAY be coming to their network when the iPhone 3G was released in order to try and keep customers from leaving and requesting PAC codes.
In my opinion the iPhone 3G will never be on any other network except O2. There may be a possibility for the next version of the phone being released on another network, but I even have doubts about that considering O2 and Apple signed an exclusive contract which supposedly runs out in 2012.