Few days before I posted about this rumor here Windows Phone 8 including screen capture functionality (Rumor) . Now it was confirmed .
Recently WP7App.de have been able to confirm the feature works as promised, with a click on the Start and Camera button. Pictures are stores in a Screenshots folder in the picture hub.
This we can do in the emulator also, with the F2 key {Start/Window} and F7 {camera} keys.
Here is this video