I have an arcade button on the back of my control panel wired to the PC where the power button normally connects to the motherboard. I fitted a relay to the power pass-thru port of my PSU so it only supplies power to that outlet when the computer is powered up, and my monitor, amplifier etc all plug into that.
So pressing the power button turns everything on. I run Linux on my cab and pressing the power button just turns the computer (and monitor etc etc) straight off. I use the ext3 filesystem and have had NO problems with data corruption etc (although I never press the power button when there's disk activity, that's asking for trouble!). The main problem for me is it never saves any settings you make (like last selected game in the menu) when you turn off.
However with ACPI support compiled into the kernel, pressing the power button signals a system shutdown, so instead of just a hard poweroff it shuts the system down first then powers off. I'm recompliling my kernel as we speak to add ACPI support - better safe than sorry!
Cheers,
Ben