My 2012 GS is currently (9/25/14) at the dealer for this same problem. It started at approx. 5k miles and is getting worse as the miles increase (not at 12k). The driver's seat rocks every time I come to a stop and also when I start back up again. It is the same problem that I had with my C5, although I don't know if the solution is the same (replace the plastic sliders with metal), because I have not seen it discussed here. I'll update the forum when I get the car back from the dealer. Nobody has complained about the passenger seat in the GS yet, but the C5 had problems with both seats.
John