From Microsoft:
Power Cover
The battery that you can type on.
Click in Power Cover to get the speed and comfort of a classic keyboard along with a boost of extra power for your Surface. Power Cover features a QWERTY mechanical keyboard, a full row of function keys (F1–F12), Windows shortcut keys, media controls and a touchpad.
Power Cover lets you take Surface from tablet to laptop and back again with a quick flip. But you also get up to 50 percent extra battery life to help you make it through that late-night meeting or the entire flight. When closed, Power Cover helps protect your screen and shuts down your display to preserve battery life.
Specs
Weight 0.55 lbs
Supported platforms Surface 2, Surface Pro, Surface Pro 2
Dimensions 7.5” x 10.9”
9.75 mm thin
Available colors Black
Keys Activation: moving (mechanical) keys
Layout: QWERTY, full row of function keys (F1–F12)
Dedicated buttons for Windows shortcuts and media controls
Interface Magnetic
Sensors Accelerometer
Warranty One-year limited hardware warranty
$199.99 ERP
Projected release date of early 2014.
I'm having a hard time believing it will weigh .55lbs which is very close to what the type cover weighs. The last rumor I've seen says it will end up weighing closer to a pound.