There are a growing number of reports about issues with the iLife '11 upgrade installation for iPhoto '11. One problem reportedly occur when converting the older iPhoto '09 libraries, hanging the program; the other presents blank images in the library instead of the photos.
Among other problems are people finding that some or all of their photos have disappeared after the upgrade. It is likely the upgrade has not deleted the photos themselves, but instead the library no longer properly points to the photo files.
The 9 to 5 Mac site offers some suggestions to repair the problem.
Of course, it goes without saying that users should always back up their system before upgrading a major software suite.