While cool, doing this actually wastes energy and drops your mileage. The reason is you loose a lot of energy discharging and charging the battery. 50% efficiency has been put out there. Add to that the efficiency of the gas engine when it eventually has to recharge the battery - around 25%, and you can see it's a pretty big issue.

The engine and cat must have been warm when you did this, as the engine will usually start as soon as you turn the car on if it's cold.

Pearl is a
2007 Driftwood Pearl Prius
Package "B" (everything but leather, nav, and rear camera)