The pessimist inside me worries about whether this could be used to lure people into revealing their credentials; granted, there are thousands of ways to do this easily, but I'm still cautiously optimistic on this topic.
Someone setup a fake tournament website that asked the user to login with their Steam account. Then it launched what looked like a new browser window with the Steam login page, but was actually just a popover that had been elaborately styled, with window decorations and all.