We've used a serial connection between the arduino and the cRIO for the past two years. It's much simpler to set up and debug than I2C.
You will need a level converter between the cRIO, which is RS-232(can be a wide range of voltages, IIRC the 8 slot cRIO was +10, -5), and the arduino which is TTL (normal 5V logic).
I used this (
http://www.ebay.com/itm/MAX232-RS232...-/221022603873) level converter board to plug directly into the cRIO. The level converter was powered with the 5V supply on the arduino, and works very well.
If you're interested, I can probably dig up the code for this.