I'm not really sure, but I think you're confusing the power on and wake up from sleep functions... AFAIK you can "turn on" the computer with your mouse or keyboard if it's in sleep mode, but not if it's powered down—and of course, disconnected from any power source! :P
Actually, the cordless mouse is powered by batteries so it has a power source that's 'virtually' connected to the computer even while it is powered-down. A cordless mouse with a PS2 transceiver can certainly power-on a computer remotely if there is a provision in the software set-up to do so. I've never had to enable it in the BIOS so I'm not positive if the MOBO must allow it too. Most modern MOBOs have an electrical current running through them unless the PSU is turned off or unplugged. Some MOBO LEDs are lit even if the computer is powered-down.
I've powered on wirelessly for over 6 years, with 4 different motherboards and 3 different OS's (XP, Vista & Win7) using my Logitech PS2 cordess trackball/mouse by just left clicking. My bedroom cordless multi-media center would be a pain without this function. IMO a multi-media computer should power-up remotely as a matter of convenience to the user.
USB mice and keyboards on the other hand are problematic. I'm still unsure if it is possible to power-up using a mouse or keyboard with a USB dongle/transceiver. It seems that the cordless industry standard is going to USB dongles and a cold power-up function would be ideal especially since so many homes are using computers as the hub to their multi-media centers.