I'm using an Arduino Ethernet, and have found that it is impossible to digitalWrite() to a pin with the Agentuino library loaded. The code progresses, the write apparently occurs, but with no voltage shift. I've tested this on 1.5.4 and 1.0.5. I've filed a bug, and reached out to the author, but this seems like a pretty fundamental flaw, and was wondering if anyone else can confirm it that has an Arduino Ethernet, or better yet the Ethernet Shield. Basically add the code from http://arduino.cc/en/Tutorial/Blink to Agent.pde (shifted to pin 6 per note below). I also added a Serial.println to confirm this behavior.
If you are using an Etherenet interface you are probably using pins 10-13 for SPI. The "Blink" example uses Pin 13 because that is where the built-in LED is connected. Try it with a pin that does not conflict with SPI.
Aah fair point, I shifted my code to pin 6 because of that, and forgot to note that in the OP. I apologize for my lack of clarity.