For US based customers, AT&T Mobility will offer the new iPhone without a 2 year contract for $599 for an 8GB and $699 for a 16GB. [Link - No-contract iPhone on the way]. But the article reports that the iPhone would still be locked to the AT&T network — so some sort of Jailbreak would be needed for those looking to buy and use a different carrier.
In the UK, it looks like o2, the exclusive iPhone carrier, will have a Pay-as-you-go option for the 3G iPhone as well. [Link o2 PAYG iPhone page].

