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Are only the thumbnails appearing grayed out? What happens if you double click on the photos? As far as troubleshooting this issue, I'd like you to follow the directions in the following article:
a best practice to is to never have any computer program (including iPhoto) delete the photos from the card but to import the photos and keep them and then after at least one successful backup cycle has completed and then reformat the card -- I use three very large (32 GB) cards in rotation so I do not reformat for typically a year or more giving me one more long term backup of my photos
Back up your iPhoto library, Depress and hold the option (alt) and command keys and launch iPhoto - from the resulting first aid window repair permissions and if necessary rebuild your database