In Lion, you can access Quick Look from Stacks without ever opening Finder.
Click on a stack in the dock. Hover over an item, and press space. A popup will appear with a preview, similar to Quick Look from Finder.
Click for a screenshot.
This works in Fan and Grid views.
[crarko adds: I recall this as being an asked-for feature in Snow Leopard. Nice to see it's there at least to some extent in Lion.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110927204301998