Thank you all who responded.
I almost went with the "it's normal" storyline until I called MS and they unambiguously told me that no, it is not normal, and I should have it exchanged. Which is most likely what I will to tomorrow, or get a refund. Not sure yet at this point.
My main concern is the fact that it was a gradual onset, it wasn't there from the beginning. Had it been there from day one, and stayed the same, it would have been one thing. But it was firm at first, and the play only increased with time. What will it be like in 2 months? A half a year from now? I'd rather get a refund while I can, as opposed to exchanging units ad nauseam.