I find this super useful and so I thought I would share. (source: http://www.thecustommac.com)
Enable See-Through QuickLook Folders
OCTOBER 7, 2009
Open Terminal and type in
defaults write com.apple.finder QLEnableXRayFolders 1
to restart the Finder. You should now be able to quicklook any folder that has files and see it in the folder preview.
UPDATE: If you ever want to disable it this feature, quit the Finder, then repeat the above Terminal command, but change the 1 to a 0.
Source: Mac OS X Hints