Handy shortcut for Leopard’s odd-behaving Quick Look feature.

Command-Option-Y will Quick Look files directly in full screen. Great for viewing images (takes a sec to load).
Esc goes up a folder while in full screen. To exit from the keyboard hit Command-Option-Y.
If you Alt-Tab to another program, then Alt-Tab back to Full Screen, you’ll be able to arrow to the adjacent files in the folder. Not sure why you can’t just jump files to begin with. Again, it’s slow to load the previews, but once they’re loaded, arrowing back and forth is pretty quick.

Still looking for a snappy full-screen image viewer like ACDSee 3.1 i use on Windows – preferably something free.

Update:
Command-Y also exits Quick Look in either full screen or regular mode (the Option key is not necessary but works as originally noted).
When navigating files using Quick Look, it is quicker to use the Up and Down arrows as that directly selects the adjacent file. Using the Left or Right arrows will trigger a flip on the Cover Flow view (in the background) which usually takes longer.