Quote - (shihchiun @ Jan 3 2008, 19:54)

In your BIOS you should be able to set options for "S3 Wake." One of my motherboards lets me use the keyboard, the mouse, etc. Then, you put Vista in standby, and when you click the mouse or use the keyboard the computer will come out of standby and you can use it. It doesn't work if you actually shut it down, because then there'd be nothing to detect a keyboard press or a mouse click.
You can turn on the computer from a complete shutdown but your BIOS has to support it. I can turn on my PC by hitting the spacebar on my PS/2 keyboard, not sure if it would work with a USB one.
EDIT:
Quote - (shihchiun)
because then there'd be nothing to detect a keyboard press or a mouse click
Think of Wake on LAN but Wake on Keyboard