Tesco Mobile has added a special SIM-only contract for the iPhone 4 and iPhone 4S with a three month contract, which is possibly one of the best offers on the market, especially if you're a light user.
For £12.50 per month, users get 500 minutes, 5000 texts, 1GB data and unlimited BT Openzone Wi-Fi access, which is, by all accounts, a fairly impressive deal.
Opting for the exact same tariff but on a one month contract with Tesco will cost you £20, which means that it is cheaper to stick on a 90 day contract (£37.50) than a 60 day contract (£40).
Compared to O2, which is the infrastructure provider for Tesco, the Tesco offer is a bargain; 600 minutes with unlimited texts and 1GB data on a one-month contract would cost £31 per month with the former. Against GiffGaff however, it's swings and roundabouts.
The other O2 MVNO has a £15 deal which lasts only one month, comes with 400 minutes, unlimited texts and unlimited data access but no Wi-Fi, making it ideal for heavy data users who might feel constrained by the 1GB limit on the Tesco package.
Note that the Tesco microSIM comes with a SIM converter to fit the SIM into other smartphones. Historically it has always been cheaper to buy a SIM Free, unlocked iPhone handset and the SIM card separately.