OS X 10.5’s numerous changes is a subtle one in the behavior of Preview’s drawer. In OS X 10.4 and earlier, if you dragged an image out of Preview’s sidebar, it would vanish in a puff of smoke, much like when you remove something from the Dock. However, this was a basically meaningless action—nothing would actually happen to the file you dragged, other than seeing it vanish in a puff of smoke. In the Finder, the original file could still be found in its original folder. If you wanted to move or copy the original file, you could do so, but you had to use the proxy icon in the window’s title bar.
In 10.5, though, Preview’s sidebar images are now connected to the source files. When you drag a thumbnail from Preview’s sidebar to a folder in the Finder, you’ll create a copy of that file in the dragged-to folder. The original file is not moved, and there doesn’t seem to be any combination of modifier keys to enable a move—you’ll always make a copy when dragging from Preview’s sidebar.
Still, if you use Preview to scan a number of images and sort them into “keep” and “recycle” bins, for instance, this new behavior can be a timesaver. Just drag those images you wish to keep into your Keep folder directly from the sidebar, and you can do all your reviewing and sorting directly from Preview.