No, you dont get charged when you just activate it with an AT&T sim.
If you made a phone call with that sim then it will just deduct that phone call or minute of your plan thats all.
If you make a call with the card in your phone and then yes you will be charge (or minutes will be deducted).
Otherwise, like Applejuiced noted you will not be charged for activating the iPhone unless the SIM card is new and you are activating a new line which then you may be charged activation.