Finally! Its taken Google until v4 of Android to have a native screenshot capability, which doesn't require rooting or connecting via USB and running up the ADB part of the SDK.
Its a bit of a knack, what you do is press the volume down button and the power button _together_ for about a second.
The current image on screen will shrink, and then you'll get a menu bar notification. Use your favourite tool eg Bluetooth to move the screenie off your device.