Having a little network background, the first thing that comes to mind is use it as a Snort box, and then you don't have to worry about only having one interface. You would setup your switch/router to forward all traffic to the Pi, but also pass it onto the end device. This is known as a switchport monitor, but your device may not support it. You may need a managed switch to do this. Snort would then analyze the traffic and report on it. More information on Snort http://www.snort.org/
The nice thing about this kind of setup is your Pi is sitting off to the side watching everything that is being run through the switch, rather than sitting inline with the data. If you have a software glitch this way, traffic can still flow, just not be monitored. With what you are describing with two interfaces, your program crashes, your network crashes.
This does not use the pi as a router, but does allow you to monitor and report on traffic that is moving across your network.