Guess what? For some reason, the iPad uses a smaller SIM card than your standard cell phone. (Engadget explains it all here). So, if you're an iPad owner (or reside in the UK - see below), you may find this tutorial by John Benson pretty handy.
"This morning, my shiny new iPad 3G 64Gb arrived from the USA! The only problem was, it had an AT&T MicroSIM and as yet there is no such thing in the UK.
So what's the solution? Get a chopping board, a meat cleaver and a pair of scissors - simple!"