O2 has posted prices for their Pay & Go service. The new iPhone 3G costs 299.99 and 16GB for 359.99 for Pay & Go service which does not require a long term contract. This price includes unlimited browsing and Wi-Fi for the first 6 months after you activate your iPhone, but does require a 10/month plan to continue after that time.
The 299.99 price is much lower than the prices that has been previously rumored for unsubsidized iPhone. For reference, the original iPhone was priced at 269 and required an 18 month contract.
Update: O2 has removed pricing from their website.
Related Forum: iPhone
