Jason on his blog (http://www.boeeerb.co.uk/pitx-an-atx-style-solution-for-the-pi/) described an innovative approach to controlling the power on a raspberry Pi.
I've already corrupted the file system on my Pi by just pulling power, so this was a great idea!
Here's my first prototype based on Jason's work. This version uses a DC 2.1 barrel connector to a 5V 1A wall wart, a DPDT 5V NTE R40 relay, some tactile buttons from adafruit.com all soldered together on an adafruit perma-proto board.
The large tactile buttons are color-coded green for go/start, red for shutdown/stop. The shutdown button is hooked into GPIO 18 on the Pi, where a running python script checks for signal. On high, the script executes a shutdown.