How much power does Apple TV use on standby?
According to the review on FlatpanelsHD.com, it will use 0.8 watts when in standby (not sleep), and 1.9 watts when in use. To use their description, this is "ridiculously low."

Since all the device needs to do is wait for Bonjour, AirPlay, or IR signals — and the upper number matches up with Apple's own report — I'd say this is believable.

Fun perspective fact from the Apple report: It takes 30 Apple TVs, all streaming HD video, to use the same amount of power as a common 60W light bulb.

You can quickly put the Apple TV to sleep by holding the play/pause button on the home screen. The cursor should be over one of the apps, not a content preview, and last I tested, this didn't work with the play/pause button of the Remote app.