I am having a very perplexing problem. I am trying to using pins 22-27 for the Hayesduino project (http://hayesduino.codeplex.com -- all the code is there). Unfortunately, of these pins, the only one I can get to go low from my software is pin 23. None of the other pins will go low. If I move my pins to 2 - 7, everything is fine. Please see the ModemBase.h file for the pin descriptions for both the UNO and Mega. Again, everything is fine on the Uno, and if I use the Uno configuration on the Mega. I want to leave pins 2 - 7 available on the Mega for other shields I would like to add besides just the Ethernet shield. Any guidance would be greatly appreciated.
Are you saying that
pinMode (25, OUTPUT); digitalWrite(25, LOW);
Does not work on a Mega?
Have you tried other code using those pins? Maybe just load Blink and change the pin number- that way at least you'll have the world's most basic sketch and eliminate any other potential source of problems.