Reboot works in the later builds. Not sure what Power off does, the screen goes blank and rpi is still sucking ~5W.
i think there is no hardware support to switch off the device, how debian reacts here? a halt or poweroff only stops the OS.
There is no hardware power "off" although there might be some way to return to the "unbooted state" with no video out, no SD card and no network, in which current draw is ~150 mA instead of the normal 350 to 550 mA fully operational. Not sure. Anyway if it takes 5W, half of that may be due to your 5V adaptor, or external USB peripherals. By the way, a lot of older DVD players take 5W or more when they are turned off.
If you really want "off" you need a switched AC power strip or switched USB source, like this: http://www.amazon.com/Satechi-.....B0051PGX2I