The 8GB 3G iPhone will be available for £299.99 and the 16GB version will cost £359.99. This price includes unlimited Wi-Fi for the first 6 months.
There are three different packages depending on how much you top up: £10-14, £15-29, and £30+.
[Update] O2 seems to have removed this information from their site. It is possible that it was not supposed to be made available yet!