How to use Ardunio to listen on i2c bus mastered by 2nd Arduino?

Here's an example where the Arduino listens on an I2C bus.

http://rascalmicro.com/docs/basic-tutorial-controlling-motors.html

However, reading a couple of articles suggests that each slave on the bus is required to tell the master it's address, so that's not going to work for your case.

So you need to be a bit more cunning.... luckily, someone else has already done this. Johngineer has written an I2C Sniffer that reports the data from the bus back to the serial port.

http://www.johngineer.com/blog/?p=455

Here's another interesting sketch that scans the bus for different devices which would be useful if you did not know the address of the item you were looking at

http://arduino.cc/playground/Main/I2cScanner