Apple showcased iPad WiFi and iPad 3G + WiFi for more details check here. iPad will use a Micro SIM for 3G connectivity. AT&T is likely to launch Micro SIMs with the availability of iPad in first week of April. iPad 3G users will be able to buy an unlimited 3G data plan for $29.99 per month. For just $14.99 per month, you can grab a 250 mb/month data plan. With either of those plans, you’ll get free use of all AT&T WiFi hotspots. None of these plans require a contract, so you can go month-to-month and cancel any time.
About Micro-SIM, is a smaller version of a SIM card, Micro SIM are just 15×12 mm in size – a reduction in surface area of around 52 percent compared with the current generation SIM card format of 25×15 mm. iPad 3G models unlocked to all carriers with GSM micro SIMs so so plans and SIM cards for iPad may be available on other carriers soon.