I read an article about this at work yesterday...
AT&T can tell and does not want customers to be swapping sim cards between the iPhone and the iPad.
AT&T will first warn the customer to stop using it but if the customer does not listen... AT&T will reserve the right to put the customer on the appropriate tethering plan which is $45 for 4GB of data.
The customer will lose grandfathered status if they had the Unlimited data plan if they ignore the warning and are put on the 4GB tethering plan.
So, yes, take AT&T's warning seriously...
AT&T can tell and does not want customers to be swapping sim cards between the iPhone and the iPad.
AT&T will first warn the customer to stop using it but if the customer does not listen... AT&T will reserve the right to put the customer on the appropriate tethering plan which is $45 for 4GB of data.
The customer will lose grandfathered status if they had the Unlimited data plan if they ignore the warning and are put on the 4GB tethering plan.
So, yes, take AT&T's warning seriously...