One of the greatest things about ATX cases (and APM or ACPI) is that the power button can be controlled by software! For example, in Windoze, the power button can be made to put the system in standby, hibernate, software shutdown, or some other ridiculous function that doesn't have to do with actually killing the system power! This, my friends, gives us the joy of having to hold down the power button for about 6 seconds before the screen goes blank and the power shuts off! This is always great when the system crashes and you need to turn the power off by holding the button because pushing it once doesn't do crap! Even better, you may need to turn the thing off immediately and you can't do that because, once again my friends, you have to hold the damn button down. Even better is when you don't know the button works that way and when you're rushing to shut off the computer because of whatever reason, you keep hitting the button and nothing happens! The only thing you can do is rip the power cord out! Oh Joy!

Actually, in my experience, only newer ATX power buttons work this way. A 1997-era Pentium 166 Dell I work with never has this problem, but my December 1999 P3 Dell I own does occasionally. And at this computer at school, a Pentium III 733 MHz Comcrap, that's exclusively how it works. Braindead design at work, my friends.