If all you want is get the RFID data to PC you actually don't need netduino for that. You can simply connect it directly with the FTDI Serial-to-USB cable.
Or you can just get the USB version of the same Parallax RFID reader.
Then it will show as virtual comport that you can access using System.IO.Ports.SerialPort class (I should have some sample code if you need help).
That's good advice. And I've looked at going straight to the USB version of the RFID reader. My big motivation for using the Netduino though is that I'd like to add more then one sensor and even some other inputs (switches, knobs, etc) and have it do all the physical device handling. That way, I only plug one device into the system and get an array of data out of it.