In short, depsite accepting all your points about switching speeds etc
etc I still maintain that PC's in practice don't Idle.

Do you know about CPU Halt and Grant Idle states? Almost any modern
operating system will enter these states as soon as CPU usage drops below
100%. This isn't some setting in a property page somewhere, it happens
automatically whenever CPU usage isn't 100%.

This isn't an operating system Standby or Suspend state, where it blanks the
screen. It is an instantaneous and effective power reduction mechanism that
is instantaneously reversed as soon as CPU is required.

Christian.