@PingSpike I followed the thread @HoolyHoo is linking to above a couple weeks ago and it works great. You need to use GPIO3 if you want to be able to use the same button to turn on the Pi from idle (connected to AC, but shutdown). Also, there is sometimes a delay (polling rate for background BASH script?) so you have to hold the power button in for a second or two before the system recognizes it, but it has never failed to shutdown safely.
I'd really like to get mholgatem's GPIOneer working on Jessie so I can get my volume up/down buttons working again--no delays, any command can be assigned to a GPIO button (or combination of buttons) and web-based config editor.