Actually, the Macally iKey USB keyboard has a power button, but I'm not sure if it's still being made. In any case it won't turn on my G5, but it does bring up the Sleep/Restart/Shutdown dialog.
I have a Tactile Pro keyboard. It has a power button, too. But like others have said, it only brings up the "Special" window where you can select Restart, Sleep, or Shutdown. When the machine is off, it won't turn it on.
I think the last time the keyboard power buttons still worked was on my B&W G3. Can't remember if it worked on my AGP G4...