[Turns out this is known bug #9536151]
I have a strange issue with Quicklook in Lion. This is on a clean install and happens with more than one user. It also isn't fixed if I clean out the Quicklook cache in /var/folders
I have to push the spacebar twice to get Quicklook to work properly.
The first time the usual Quicklook 'window' appears but it is blank:
Push the spacebar again and the image appears. If I restart the Finder the problem goes away briefly. It's driving me nuts.