I tried the usual corrective measures: repaired permissions, reinstalled iPhoto, rebuilt the library and thumbnails, etc. Nothing worked.
It turns out that the culprit was a missing ColorSync profile. I found this by checking my iBook (I had installed iPhoto on my iMac), which had one more profile than the iMac: sRGB Profile.icc. This profile is located in /System -> Library -> ColorSync/Profiles. I was able to copy it from my iBook to my iMac, and everything was fixed.
This hint is dedicated to all those who struggle with insurmountable problems, harass friends to get help, curse the software, and search Apple's support site, only to find that the answer is not there...
[robg adds: I can attest that Kirk did, in fact, curse the software mightily until he found the fix. The bigger question that neither of us can figure out is why that profile wasn't on his main machine -- it's not a file one would be able to accidentally delete, given its location.]

