Further digging by Chipworks gave us the identity of Apple's mysterious M7 processor, a chip that was conspicuously absent from last night's iPhone 5s teardown. It's an NXP LPC1800 device that was buried beneath a neoprene-looking cover:
From Chipworks: "The M7 is dedicated to processing and translating the inputs provided to it by the discrete sensors; the gyroscope, accelerometer and electro magnetic compass mounted throughout the main printed circuit board. Traditional Apple techniques lead us to believe that the these discrete sensors will most likely be STMicroelectronics for the accelerometer and the gyroscope while the electro magnetic compass would again be an Asahi Kasei Microdevices (AKM). We have since confirmed the compass to be AKM’s AK8963."