Were you planning on buying an iPhone, bringing it home and immediately swapping out the SIM card? AT&T says no. Their recently posted iPhone FAQ states that your iPhone must be activated (i.e. through iTunes, with a credit card and a two year calling plan) before the phone can be used. You can't just swap out your current SIM card and start right away. Apple's Q&A adds (rather unhelpfully) that you "should use the SIM card that came preinstalled in the iPhone", giving no hint as to whether a SIM swap is possible.
As I posted yesterday, activating the iPhone restarts the 2-year calling plan clock, regardless of whether you're an existing customer or not.
Thanks, Ryan Valle.
Update via David Pogue: "Can I use a SIM card from another phone? The iPhone comes with an installed SIM card, the tiny circuit board that stores your account information and phone number. Apple says that you should be able to replace it with any recent AT&T card, once you activate it in iTunes. No other company's SIM card works in the iPhone."