Interfacing cell phone to microcontroller
You can use an Android to serial adapter like this one: http://www.startech.com/Cards-Adapters/Serial-Cards-Adapters/micro-usb-serial-adapter-android~ICUSBANDR232
or
http://www.pcables.com/

Both of these parts use FTDI's USB to serial interface.

As far as other solutions, I like using a serial to Bluetooth module (like this: https://www.adafruit.com/products/1588) and then just doing the connection wirelessly.