If I was to start a 18 month or 24 month contract with O2 when iPhone 4 comes out and next year there was to be an upgrade of iPhone 4, like the 3G to 3Gs, would they offer me the upgraded phone or would I be stuck with iPhone 4 for the entire contract?
I think you would be stuck with the phone for at least a good 18 months if not possibly the entire contract.
Best bet is if you upgrade every 12 months get the pay as you go one and then choose the simplicity tariff