Does the iPhone support dual / multiple SIM cards?
No, the iPhone only has space for one SIM card. The iPhone 4 only holds one Micro SIM card.
Creative developers have enabled dual and multiple-SIM solutions for the iPhone, however these are unofficial hacks that likely violate the Apple warranty and cellular carrier terms of service. Having more than one SIM card makes it possible to use one iPhone on multiple carriers or have two phone lines on the same mobile device.