Sorry to resurrect this thread, but I thought I'd pass on this tip for others faced with the same problem.
There's one easy way to solve this which works with most canon cameras: disconnect the power supply and force the power button down by some means (A cable tie and some small object over the power button should suffice).
When power is reapplied the camera will notice the power switch is pressed and will power up. Because the state of the power button never changes, the camera doesn't power down.