I had a similar thing happen. Walked into Best Buy to buy a case that will work with the 4S. Salesman walks up, asks me if I need any help. I tell him I'm looking for either a case for the 4S or the Verizon iP4. He says, "oh, any case will work as long as it's for the iPhone 4. They're EXACTLY the same". Um...no they're not.
So, I had to inform him that the mute switch is closer to the volume buttons on both the Verizon iPhone 4 and the 4S. The only difference between those two is the 4S has a SIM slot. And yeah, while technically a case for the AT&T iPhone 4 will work with the 4S, the mute switch will be nearly covered and may not be able to toggle the way it's supposed to.
It amazes me that these guys don't know the basic differences between phones. Or maybe I'm just expecting too much. I don't know.