Some people use professionally cellphones that can use simultaneously two SIM cards. Apple has never been interested by that (small) market.
USBFever is now coming up with an adapter allowing the use of two SIM cards in an iPhone 4.
The adapter is sold for less than $30 and allows to go from one card to the other through software. However, that system is incomplete since it can only use one SIM card at a time. It is for example impossible to receive simultaneously calls from either one of the two cards or to choose the Sim card to be used for a specific call. Also it takes between one and two minutes for the phone to go from one SIM to the other.
If you want to use SIM cards from different carriers, you will need the unlocked model, and don't forget to get a case to store the cards and the wires.
