This one's important for those of you looking to switch to the HTC One X (as well as a number of upcoming phones). You're going to need a micro-SIM, which as the name suggests is smaller than a full-size SIM card. If you're rocking a GSM phone right this second, there's a good change you're using a full-size SIM. If you've got an iPhone 4/4S on AT&T, you've already got a micro-SIM.
So, a couple of ways to go about getting a micro-SIM. One is to just ask. Head to AT&T or T-Mobile or whomever your carrier happens to be, and tell 'em you need a micro-SIM. (If they don't know what one is, it might be time to consider switching carriers. :p )
The other way is to cut your own. Sounds scary. Sounds dangerous.