I wrote a lenghty rant on this a few weeks back as part of the response on a lightning fried computer.
The bottom line is NO ATX computer is shutoff unless you have it connected to a power strip and shut off the switch on the power strip!
The front panel switch is just a momentary button that tweaks the power supply to go from very low power mode to full on. You can also turn ATX computers on from the keyboard, by using a jumper from the LAN card to the MB or with a modem access.
This means your computer is running during every thunder storm that goes by, all day, all night, you just don't realise it.
GET A POWER STRIP (with built in power surge protector and built in modem surge protector and switch) AND USE IT! I haven't seen anything for lightning hits coming in over the DSL or Cable lines yet so I'm still a little worried about my system. Note, the phone lines have worse lightning hits than the power lines.
The computer (or data) you save may be your own.
Just think about it if the RIAA spybots see you access Kazaa during the day they can come back and turn your comp on at 4 AM when you won't notice the HD activity and scan your entire hard disk for content.