One of the behaviors that has changed in Preview from Tiger to Leopard is the dragging of thumbnails from the side drawer.
In the Tiger version of Preview, dragging a thumbnail out of the drawer would cause it to disappear in a puff of smoke, but the original file wouldn't be touched -- similar to what happens when you drag something out of the Dock.
In the Leopard version of Preview, dragging a thumbnail out of the drawer will copy that file to the place where you drag it -- very useful for scrolling through lots of images and picking your favorites.
Mac OS X Hints
http://hints.macworld.com/article.php?story=200712120906404