G5 wakes up for no apparent reason
Have you tried Reseting the SMU


  The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any issues regarding these functions, resetting the SMU may resolve the issue. The functions controlled by the SMU include:
  
  
  It tells the computer when to turn on, turn off, sleep, wake, idle, and so on.
  It handles system resets from various commands.
  It controls the fan.
  
  
  Note that resetting the SMU does not reset the PRAM.




Steps to reset the SMU on:


  Power Mac G5 (Late 2005)
  
  To reset the SMU on a Power Mac G5 (Late 2005) computer, either use the steps listed above for the Power Mac G5 (Late 2004) or:
  
  
  Turn off the computer by selecting Shut Down from the Apple menu or
  by holding the power button until the computer turns off.
  Open and remove both the metallic outer door and the inner plastic
  air deflector.
  Remove the fan assembly immediately to the left of the processor
  module.
  Press the SMU reset button on the logic board.
  Replace the fan assembly, air deflector, and outer door.
  Turn on the computer.
  
  
  The SMU reset button is located underneath the lower bank of system memory slots, as shown below:
  
  
  
  Unlike earlier models of Power Mac G5 or Power Mac G4 computers, the Power Mac G5 (Late 2004) and (Late 2005) models do not have a PMU. This functionality has been replaced by the SMU.