The 3G version of the iPad is unlocked, so if your carrier supports micro-SIM, it should "just work". Wait... what? Steve Jobs dropped the micro-SIM comment so matter-of-factly during the iPad introduction that it took us a moment to process -- Micro-SIM, aka 3FF or 3rd generation SIM card. Yeah, don't have one of those, and neither does any iPhone. Awkward...!
PCMag's Sacha Seagan fills us in:
3FF SIMs were developed by the European Telecommunications Standards Institute to fit into devices too small for a regular SIM. Where a regular SIM measures 15 millimeters x 25 mm, a 3FF SIM card measures 12mm x 15mm.