The MicroSIM being used by Apple's iPad is smaller than a normal SIM, but that's easily solved with a pair of scissors, or a meat cleaver if you'd prefer.
Certainly that's John Benson's preference: he got hold of a US-issue iPad 3G, sliced down his Vodafone SIM and managed to squeeze it into the MicroSIM slot on the iPad. He reports that it works just fine.
Not that there's any reason it shouldn't - the electronics on a SIM, and in a credit card for that matter, are glued to the back of the metal contacts and hang in a void carved out of the plastic. The surrounding body is nothing more than a glorified handle, so removing it shouldn't have any impact on the function.
There's no indication that the iPad is carrier-locked either. Right now the device is not operator-subsidised so Apple has no reason to impose restrictions. That may change in the UK, where operators have hinted they'll be offering subsidised iPads, but we'll have to wait to see if those are locked to a network, and how hard that lock is to break.
So it should work, though we're not sure why Benson feels a meat cleaver is the way to go. We've cut up a few SIMs in our time, and for precision work nail clippers are the tool of choice, but far be it for us to stand between a man and his blade of choice.
We don't have an iPad handy to try this with, so we can't guarantee success. If you want to give it a go, our advice is to practice with a cheap SIM first, and stick with nail clippers, unless you don't mind losing a finger or two in the process. ®