The iPad 3 model with cellular data, iPad 2 3G on AT&T and the 3G model of the original iPad use micro-SIM cards. The Verizon version of the iPad 2 3G does not have a SIM card, and no Wi-Fi models have a SIM card.
Can the SIM card be removed?
Yes. The slot for the SIM card on the iPad 2 and iPad 3 is on the side, towards the top of the iPad and on the opposite side of the volume buttons. On the original iPad, the slot is on the lower left side.
How do I remove the SIM card?
First, locate the small hole next to the SIM card slot. Either using a SIM card removal tool or a paperclip, press the end of the tool into the small hole. The SIM card tray will eject, allowing you to remove the SIM card and slide the empty tray back into the iPad.
Refer to this Apple support document for a diagram of the SIM card slots.