I just spoke to someone at Apple who called me all the way here in Barcelona to see if my data was OK and if I needed any help. I was pretty impressed. They know how important our photos are to all of us.
They confirmed that there was indeed a window during the updating process in which a force quit could “create data loss”. I asked if I would have had the same problem without force quitting and the answer was probably not, though they didn't really specify how long I would have had to wait. There are people on the Apple Discussion Forums who have let their updates go for days with no apparent signs of life. I asked if RAM could have been an issue, and they said no.
They asked me if I had any projects created with older versions of iPhoto, and I said I had two from iPhoto 5. And they asked me if I had any video playback extensions for QuickTime (I don't). They also asked how big my library was (233gb).
I asked if changing permissions of the iPhoto Library was really helpful (as someone suggested on one of my blog posts), and was told that it was only part of the solution. Deleting old projects does not seem to have any effect whatsoever.
Mostly, and this was the most important thing, they told me that a fix from Apple was imminent and that we would soon have official confirmation of same.
It's probably best to wait for the fix, but if you can't, be sure to properly BACK UP your iPhoto Library before upgrading to iPhoto 11!