I currently shutdown my hacked plug (ARM) computer using a SSH phone app (shutdown now command) .. its a bit of a pain and the pi will suffer from this as well as it has no shutdown switch.
I was thinking about this and came across some of these <$5 usb buttons (used for advertising etc) and it looks like this or something like it could do the job .. probably needs the eeprom reprogramming though as they are preset to generate a specific string on the keypress
Over on the hacking debian dockstar forum they have used the cheap < $5 usb rubber keypad and theres a detailed debian arm instructions on the software side to get an led lit when debian is up and once you what the switch it runs shutdown now and the led goes out when debian is down .. should be pretty easy to replicate this on the pi
as this are only $0.50 -$1 in bulk in china perhaps the pi shop should have a preprogrammed pi shutdown usb smart button with a big byebyepi raspberry on it
here are a few links as i know people have been asking for some solutions