Tip: You can also drag and drop an image to initiate a search in Chrome or Firefox 3.0+. Simply click on an image, hold down the mouse, and begin dragging it toward the search box. You should see a blue box appear, and then you can drop the image into that box.
So the drag and drop is only supported in FireFox and Chrome?? Maybe the documentation hasn't been updated in awhile.
My best guess is that we're looking at some application of the new drag and drop features of the HTML5 DOM
Check out the demos:
Perhaps it is possible for a drop zone in one tab to intercept objects from another tab? I don't know for sure -- but it seems conceivable.
Also, the fact that I couldn't perform the Google Images Search drag and drop in Opera seems to support my theory, since none of the drag events of HTML 5 seem to be supported yet in Opera