Yes mine does this as well. It is very aggrivating. I have complained to the dealer about it once before and the comments for the complaint were "recommend leather care". I already clean and treat my leather with Lexol so that is definately not the issue. I believe the driver's seat does it as well but not as frequently. Over bumpy roads it is most apparant. I haven't deduced whether it is coming from the mid-back fold hinge or the base hinge, but I believe it is squeaking from the base hinge for the backrest. I'm thinking a shot of lubricant (WD-40 or equivelant) to the metal hinge materials should quiet it up, but I haven't taken the time to remove the plastic covers and do it yet. If you place your hand on the back of the seat and rock it forward and backward while pushing to either side you can duplicate the noise sometimes, which will help in determining if lubricating the hinges fixed the problem. Quiet annoying to have a 6 figure car with squeaky seats grrrrrrr.
Oh and I also have the Multi-Function Sport Seats (wonder if the problem is related to them specifically or if the base seats have the same issue).