[whpalmer4]

It wasn't working for me, either, so I went poking around. I opened up the app with Show Package Contents, then looked in Contents/Library/QuickLook and double-clicked OmniGraffle.qlgenerator which didn't appear to do anything, but when I then tried to do QuickLook on an OmniGraffle file, I was rewarded with a preview.

If you haven't launched OmniGraffle since downloading it, or if you have it in a non-standard place, it isn't impossible to imagine that the system might not know where to find the QuickLook generator plugin, or whether it is okay to run it.