Quote from the ubuntu bug (https://launchpad.net/bugs/83098)
"I've noticed that holding the Windows key while clicking and dragging with the mouse takes a screenshot of the selected area of the screen. If the user clicks on another window when trying to select the screenshot area, the focus can change to the other window and this can obscure whatever the user was trying to take a screenshot of."
There is also a video who shows the problem.
I can't confirm this. It works as expected with the latest git version on ubuntu/feisty.
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
This appears to be fixed now.