I offered this comment in response to a query in this hint's comments, but I couldn't find a hint update for 10.5.6, so here goes.
When using Quick Look in full screen mode in earlier OS versions, you could us Command-Tab to change from the Finder to another application and then back to the Finder. After doing that, you could then use the arrow keys to scroll to other files. (Otherwise, you could only switch between whatever files you may have selected prior to opening Quick Look.) This no longer works in 10.5.6, but there is a similar approach, but why this one works is a mystery.
In 10.5.6 when in full screen mode in Quick Look, hit Command-Tab and navigate to Safari, and make sure you can also see a bit of a Finder window. Click on the Finder window in the background to take you back to Quick Look; clicking the Finder in the Dock won't work. After doing this, you can use the up/down arrows again. It doesn't seem to work if you switch to any other application first, and I'd be interested in any explanations.
Of course, you can still use the other standard, documented (and foolproof, if there is such a thing?) way to move between files in full screen Quick Look, by selecting all the files you want to view in the Finder and then go to full screen view (Option-Command-Y). This is the Slideshow mode but you can move through the files easily using the left/right arrows in this case, and scroll within a file using the up/down arrows. Strangely, the above hint works here too so you can go beyond the files you selected.
[robg adds: I think you can use this trick with any application, not just Safari. The key seems to be where you click in the Finder window. If I click in the window itself, I still can't browse with the arrow keys. If I click in the toolbar portion of the Finder window, however, then I can browse at will -- regardless of which application I switched to via Command-Tab. One other side effect is that this seems to disable the Escape key as an exit for Quick Look; I had to click the "X" button on the screen to get out of Quick Look after using this trick.]

