You can force Mac OS X’s Quick Look to present a full-screen preview with the Option key. Here’s how. With a Finder window open, hold down the Option key and the Quick Look icon in the tool bar will turn into what looks like a “play” button, with a right-facing triangle inside a square. With your file selected, Option-click the icon for a full-screen preview via Quick Look.
If clicking Finder window buttons isn’t your style, try this. Select the file, and then hold down the Option key while you press the Space Bar. Again, Quick Look will offer a full-screen preview.
I just discovered this behavior this morning and thought I’d share. I tested on both Snow Leopard and Lion, and it worked on each.