As announced by Apple, the iPad tablet can support any 3G carrier with "micro-SIMs".
The micro-SIM chip inside the iPad allows the subscriber to access the carrier network. While the resulting card provides the same functions as other SIMs, the microSIM requires additional precision during manufacturing as it is embedded in a smaller plastic casing.The Size of the micro-SIM is about a half of a regular SIM card (12x15 mm instead of 15x25mm).

This way, it takes up less room which either lets the device maker decrease the device size or use the space gained in other ways, such as allowing a bigger battery. The micro-SIM is based on an industry standard called 3FF or 3rd form factor.

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How is a micro-SIM different from the SIM I have in my mobile phone now?