After a year most dealers will tell you that it is not their responsiblty. The rotors are fairly inexpensive. But if you took it in for "shoes" what do the rotors have to do with it? If you have shoes, then you have rear drum brakes. If you need brake pads on the rear then you have 4-wheel disc. But I would say that after a year, this is going to be out of your pocket, sorry about the bad news. However as I said brake parts for the Grand Am are fairly cheap.
As a side note, I am not sure where you live but in Camada for 2004/2005 and up almost all GM cars came with a 160 000kms (100 000 mile) warranty. BUT I know in the states Rotors are not covered by this, but in Canada they are?? There are a few other things (but they dont apply to your situation)