Micro-SIM is well known as 3FF — The «Third Form Factor» SIM is a mini-version of the normal SIM Card and will be used in a number of mobiles soon. The actual size of the card is 12mm x 15mm which is 52% smaller than the «normal» SIM and of course it will not fit into your current phone.
The credit card-sized SIM was generation one followed by the present standard sized SIM and now the third generation SIM - the 3FF, mircro-SIM or Mini-UICC is approaching. It is much smaller, even though the contact-area is the same. Actually not many manufacturers and service providers are using them yet. The IPad will be the first to start the new aera. Many Network Carriers like AT&T, T-Mobile in north america, Orange, Vodafone and O2 will follow in other parts of the world soon. Simyo in Germany is already asking the customers to preorder the microsim sized Simcards. Other Carriers like A1 and Bob in Austria will have them available online from the 15th of April on.
Still the big question is, why is Apple using new Micro-SIM in the iPad — a device already at least double the size of an iPhone? Hardly to save space. But they will also use it for the iPhone 4G or even the built in modems in the new Mac Book Pro.
ETSI - European Telecommunications Standards Institute developed the card to fit into tiny devices — which the iPad can hardly be referred as. Anyway, for all who want to use their micro-sim in an other device as the IPad, there is now a way to do so. The micro-SIM-adapter (3ff-adapter, mini-UICC-adapter) has a frame which will carry the micro-sim (3ff-sim) and converts it into the size of the standard SIM. You can then use your micro-sim from the IPad and the iPhone 4G in your IPhone 3G.
Or if you decide to cut your standard sized SIM from the Iphone (or any other mobile Phone) to the size of the micro-sim, you could use your Phone SIM on your IPad without having to buy a second data plan. With the microsim adapter you then can use your small SIM again with your present mobile. Guides on how to cut your standard sized 2nd generation SIM to the smaller version Micro-SIM can be found on the Internet.