A quick look at the handy product page that iFixit linked to (http://www.atmel.com/devices/atuc256l3u.aspx) shows that the controller has "25 touch channels", as well as supporting Atmel's "QTouch Acquisition" in hardware.
As microcontrollers go, it's pretty beefy and would have made a decent computer all by itself not too many years ago. It's a 32-bit CPU running at 50 MHz, sporting 256 KB of flash memory and 32 KB of SRAM. Granted, it's a bit low on memory for a general-purpose computer. Maybe 20 years ago, then. :)
It sure has a lot of I/O and other nice peripherals, though.
Times have changed. Today we have full-sized ARMs loaded with binary blobs that independently serve the baseband, i.e. RF communication, of your smartphone. They talk to the application processor on a very very high level, to the point where this baseband chip has to support IPv6 natively for you to be able to use it. They are completely closed source, there is no open source alternative, and they already implement features like GPS geolocation that you do not control but directly impact your privacy. Another example would be the "silent SMS" that law enforcement uses to ping your device and get your approximate position from the tower. That chip will never inform the application processor of this, even though I'm told it has the knowledge.
Now I don't want to be paranoid, but I generally prefer if the only general purpose computer in my computer is the CPU. It's probably hopeless by now, since every other network, graphics chip and controller runs on some ARM microcontroller variant, but to see them alone on the PCB makes me wonder what their purpose is.