One of the new bits of iPad viscera announced by Steve Jobs, besides the A4 chip, is that it's using a micro-SIM card for its 3G powers. What, pray tell, is a micro-SIM card?
It's the next generation of SIM card—3FF or third form factor—which is physically smaller, with less blubber around the shiny gold contact area, as you'll notice in this helpful picture from Wikipedia. The contact area, where the technobloop happens, is the same size, so it's backward-compatible with a physical adapter. But it fits in more stuff! Like a bigger internal phonebook that'll has room for emails, and a new authentication scheme that fights carrier spoofing.
Because it's the same size and stuff electrically, there's a chance you'll be able to shave down regular old SIM cards—so they'll physically fit in the slot—and use them in a micro SIM slot. Though we'll see how well that works when we get our hands on some stuff—like an iPad—to play with. [PC Mag, Engadget]