I have this same issue... (about 0.5mm lower on the right side, and generally dipping beneath the level of the casing on the bottom of the trackpad compared to the top). Went to an apple authorized service centre, the guy said that it was "normal" from wear and tear. If I really wanted to, he would need to order a new trackpad (even though I've been told by members on this forum that a screw needed to be tightened).
Although I have applecare, he refused to tighten the screw because "apple would not pay him unless he ordered a part" - which meant he had to order & replace the trackpad or the whole top casing, and hold my laptop for 4+ days (without giving me a loaner) while the parts arrived.
Is this the apple service I've heard such good things about???