The iPhone 4S is now available from as little as £27 per month both on Vodafone and O2 although you will have to go through Carphone Warehouse's Mobiles.co.uk website.
That said, the smartphones are actually refurbished (which means no glossy box and no accessories) but since they are from Apple, we expect them to come with a one yer limited warranty (with the option to buy Apple Care's Protection Plan) and to have gone through the company's rigorous refurbishment process prior to sale.
The phone was launched in October and retails brand new for £499 at Apple with the cheapest "brand new" contracts starting from £31 per month.
Buy the handset (a 16GB, black model) through Topcashback and you could get yourself £30.30 cashback which brings the total cost of ownership to only £618 or £25.75 per month.
O2's deal comes with 200 minutes anytime any network, 500 texts and 500MB data while Vodafone's offer includes 100 minutes, 500 texts and 250MB data plus Vodafone Passport.