I just finished integrating the code to access the Tiny-GPS (https://github.com/wertarbyte/tiny-gps/) module via I2C.
Tiny-GPS is a firmware for the ATTiny2313 microcontroller which turns it into an I²C-GPS device - reading NMEA data from a serial GPS receiver, interpreting it and passing it to its I²C master.
The data is pulled over in a comfortable struct, allowing easy access to all relevant data fields.
It's probably the cheapest way to connect an old GPS receiver to your copter: most USB receivers are in fact serial receivers with an integrated Serial2USB chip (pl2302 or FTDI), so if you have one lying aroud, try prying it open. You can get ATTiny 2313 controllers for 1€, and you can program them using either an ISP programming module or by using your regular Arduino device.
Here are my changes to the multiwii firmware: https://github.com/wertarbyte/multiwii- ... ts/tinygps
Feel free to merge, patch and give feedback.
