I didn't think about the shutdown part, just the startup. You're right, only one button to press.
Abother solution is to use a UPS that will tell the computer to shutdown (properly) when power is lost and put all other elements (marquee light, speakers, ...) on the non-backed up outlet (300W APC UPSes are available free after rebate once in a while)
Yeah, that would work well as long as you have an OS running that can detect and use the UPS. (but if you don't, you probably don't need an orderly shutdown anyway.

) My UPS is good about shutting down the computer in a configurable way (can wait x number of seconds, you can specify). But if you had the UPS unplugged for a long amount of time it might lose its charge. I don't know if a completely uncharged UPS requires warm-up charge time before it lets the computer turn back on. They might shunt line power to the PC when it is available and charge the battery separately, or they might continuously charge the battery while using the battery to power the PC.
I still think switching a relay off of the 12v line in the PC is the way to go. Now that I have some components other than the PC to power up/down with my machine, I'll be doing that soon.