Probably a scam, as I currently work for O2 on the legal side of things I know that such a clause currently does not exist.
If a customer was to break out the contract they would be requested to pay the remainder of the contract, which can vary from £400 up to £1,000.
If your really wanting an iPhone with no contract, wait until christmas for the pre pay one OR get a handme down iPhone 2G and use that with your current contract/pay as you go.