just so were clear, the manufacturer warranties the device. The carrier is responsible for the air time and data usage.
What AT&T is doing is acting on behalf of the manufacturer and looking out for their best interest to keep you as a happy customer.
This is why its called a manufacturers warranty.. Returning to the POS (point of sale) aka AT&T store is where HTC is aughtoizing ATT to exhance the unit on their behalf.
but im glad to see your able to get it taken care of. Thats what really matters.
To get back on topic. I generally see data cables fail more often then not when it comes to the data ports on phones. When people come to me saying my phone stopped charging they almost always seem to charge fine with our cables. Then ill have the customer bring the cable if they have not done so and test with theirs, and sure enough its a data cable issue.