Apple Support Discussions user "CamButters" reports:
I'm also getting blank previews in Finder and Quick Look. I just restored my system from time machine about 2 hours ago. I had messed up my permissions so bad that the computer would get stuck at the load up screen. After restoring, everything seemed fine except Quick Look and Finder Previews are blank.
Users experiencing issues with Quick Look have attempted various standard troubleshooting fixes including repairing permissions, deleting Quick Look preference files, relaunching Finder, and creating a New Users Account. These solutions did not produce results. However, a tried and true troubleshooting technique seems to have resolved the problems. Download and install the Mac OS X 10.6.2 Snow Leopard Combo Update package from Apple. This update contains all the information necessary for 10.6.2 to run properly on your machine--information that it seems Time Machine backups failed to restore in regards to Quick Look.