Modern (ATX or ITX form factor) PCs have a feature known as soft power, where the power-supply provides a 5v, low current power rail at all times (as long as it isn't unplugged). If the power-supply finds its 'wake up' pin shorted to ground, it powers up the rest of the power rails. This is A Good Thing, for several reasons, both for the end-user, and the system builder.
The most important advantage over AT is that all the high-voltage components are inside the power-supply. This means that if you have a grounded, fused power source (such as the British fused three prong plug, or the American three prong plug connected to a fused outlet), there is no chance of electrocution; any high-voltage components will short to the grounded power-supply case (and hence blow the fuse) before there is any chance of someone touching them. Compare with the older AT power supplies, where a high-voltage wire leads out of the power supply to the front of the case, to the front-mounted power switch. There is no protection if the live wire were to detach and touch the case, especially if there is no continuity between the front panel and the power-supply.
Having the power-supply controlled by the motherboard allows the PC to do new things, including wake-on-LAN, where the network card is kept powered, allowing the PC to be booted remotely over the network; wake-on-alarm, where the PC can wake up at a scheduled time (to defragment when no-one's home, for instance); and, of course, to switch itself off when the OS is shut down, rather than stick on the it is now safe to switch of your computer screen.
The AT's power switch is emulated by the soft power switch, which is typically a SPST Push to make switch. For AT form-factor motherboards with ATX power connectors, it is possible to buy a switch that has the same dimensions as the AT power switch, allowing soft-power to be used in an AT case. On most motherboards, holding the soft power switch for 2 seconds forces the power supply to switch off, while simply pressing it passes a signal to the operating system, which may perform a special action, such as hibernate. Some motherboards can be set to force a switch off the moment the switch is pressed.
If you really must have the power supply controlled by the front panel and only the front panel, it is possible to disconnect the PS-ON wire (pin 14, green) from the ATX plug, and wire it through a switch to any ground wire (pins 3,5,7,13,15,16,17, black). While this will allow the power supply to be switched on and off manually, it will probably produce strange results with most ATX motherboards, as they expect to be able to control the power supply themselves, and may not boot unless their soft power switch is used1. It should allow an ATX supply to be used with an AT motherboard with no problems.
I have not tried any of these hardware hacks. If you do something you read on the internet to your expensive PC, you do so at your own risk.
1 - shallot has tried this with a motherboard that could no longer switch itself on. It booted fine after the power supply was started manually, and had no problems with 'not being ready' to be switched on. Same caveat applies - if you intentionally short pins on your power supply, you do so at your own risk.
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