As can be seen from Apple’s Leopard presentations before release, as well as on page 22 of the Leopard manual, opening a Keynote presentation using Quick Look should allow you to see slide thumbnails on the side, as in 0:28 in this video. However, this is not set as the default behavior. Achieving this is very simple, though. In Keynote’s preferences, under General, simply check off “Include preview in document by default.” There you have it!
Keynote Thumbnails in Quick Look