Just to be sure that we're talking about the same thing, are you referring to Preview or QuickLook? When you hit the spacebar, you invoke QL. Preview is Apple's PDF viewer and you would need to actually launch it to see a file.
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Jon
Mac Pro Quad 2.66 GHz, one 500 GB Hitachi HD, three 320 GB Hitachi HDs, 5 GB RAM, OS 10.6.4
Epson R2880, LaCie 80 GB FW drive, second internal DVD drive (Pioneer), Photoshop CS5, Office 2008,
Nikon LS 8000 scanner
Apple 23" Cinema Display