I am thinking about building a washing machine sensor, similar to the one in Ars Technica, but using an Arduino rather than an Electric imp. I will link this to OpenRemote and can then fire a rule when the washing machine stops.
I will probably use an Arduino Uno, an RF 434Mhz transmitter and a Mindsensors NXT acceleration sensor, which I rarely use.
A similar sensor would probably work on the dryer as well. I can either produce two devices, or one device with two acceleration sensors on fairly long leads, or I can move a single device from the washing machine to the dryer when I move the clothes.
The Mindsensors device is a 3-axis accelerometer, but only one axis should be necessary for this application. I will look out for cheap acceleration sensors.