As with all products, the iPhone will eventually become old, loads of people will have one and O2 will start discounting. If you want one of the first then you pay for it. Simple as that!
I don't see there being much room to discount. Just look at how Apple keeps the iPod artificially at RRP no matter who sells it. Even online stores charge virtually RRP for the iPod range.
The $200 drop in price in the US was a shock. I would expect Apple to follow a similar business model to their iPod range in future though. You'll see a fixed price for the lifetime of the product and then it'll be replaced by a new product at a new price tag. No discounts. One price from every shop and for the lifetime of the product.