I've made one of these just this minute from an old USB iPAQ Hotsync/Charging Cable I found in the loft with a switch that changes between data and charging. I've just cut the end off and replaced it with a Micro USB one.
There are 5 wires inside, I've cut off the 3 that relate to "hotsync" and just used the red and black ones for the power. So when I switch it to hotsync the Pi turns off
I've just bodged it together for now so it's about 3 meters long lol, but I intend to cut it down to a shorter length and tidy up the join at some point. It's quality because it just happens to have a red led on it when it's off (in hotsync) and a green one on it when it's on (in charging)
Works really well using an old 5v 1A USB mains plug that I got with my HTC Touch Pro 2