And as expected, Mr. Gruber downplays this terrible, terrible bug. http://daringfireball.net/linked/2010/10/25/castro-iphone If Microsoft (or Adobe) released an update to the software which wiped out data as precious as your personal photos, I am sure he would not talk about backups, but thrash the company.
I upgraded two libraries to iPhoto 11 without a single issue. One was 40,000 pictures and the other was 5,000.
Chances are it didn't manifest itself before.
For what I know of Steve's reputation, I wouldn't like to be in the iPhoto team right now.
But then, I would always keep the photos as separate files on a directory tree with the metadata in a single file. This way, the chances of a catastrophic data loss would be very slim.