I am having a similar problem. My hard disk had to be replaced. I didn't have my old iPhoto installation disk/software so I purchased the lastest version 9.4.3. After installation, I copied my iPhoto library from the newest file on my time machine to my hard disk and then imported it into iPhoto. IPhoto prompted me to update the library, so I did. The majority of my photos are there and in good shape. However, I've found so far 13 events where the key photo as well as all the photos in the event are a blank grey thumbnail. I can view the photo info, but not the photo itself. If I scroll away from those event and then back the thumbnail pic will flash for a fraction of a second, but then back to the grey.
Things I've already tried:
rebuilt the iPhoto library 4 times (I read sometimes it take 3-4 times)
repair the thumbnails 4 times
plus the other two options in the repair/rebuild section, once each
searching for the file name on my computer via spotlight
exporting the photo onto my desktop
None have worked yet. Any solutions?