Thanks Old Toad for your long service to these forums!
With that said- yikes- I started having the same symptoms as the original poster yesterday! Viewing an Event, and while scrolling through the images, the pictures suddenly all disappeared! The title bar remained correct, and the photo count still said "72 items" I clicked the Return Arrow in the top left of the Events pane, then clicked back to the same Event. This time double yikes- still no pictures and it now said "No Photos"!!!
I quit and relaunched iPhoto, and like the OP, the problem was gone. My Event and Photos were back. I should point out one additional data-point. The photo that I was viewing when the problem occurred was now Duplicated. Very strange.
I located this thread and after backing up my iPhoto Library, I started with Fix #1. Alas, I was seeing the same behavior again almost immediately.
I then went to Fix #2. Big problem. iPhoto quickly advanced through "Repairing Library Database" and said "Rebuilding Library: 28887 Versions Restored". Unfortunately, for the next 8 hours nothing else happened except the spinning gray spike-wheel. I saw in another thread that this was the point to Force Quit iPhoto, so I did. Not surprisingly, the iPhoto Library was no longer usable and iPhoto launched with zero photos. Of course not worried yet since I had that backup.
I then Deleted the corrupted Library and restored from my Backup. Also redid Fix #1. When all done, iPhoto launched just fine. Basically was back to same state as a day ago, though with many hours lost accomplishing the above. At this moment, I have NOT seen a reoccurrence of the original problem. But as the OP said, this thing is maddeningly unpredictable. It's also not at all clear if it is merely "cosmetic" or if it is a problem that will eventually get more frequent and/or result in Data Loss.
I should also point out that I had been running iPhoto 11 9.1.3 and OS 10.6.7 since they both came out. Yet I only noticed this behavior starting yesterday after installing the follow updates via Software Update:
Security Update 2011-003
ProKit 7.0 (recommended for all iPhoto users)
Digital Camera Raw Update 3.7
I should also point out that after installing those updates, there is definitely a different Startup behavior when launching iPhoto. iPhoto launches to a blank Events pane and remains in that state for 10 seconds or so. No spike-wheel, no beachball, just nothing. It's really kinda scary and looks like an empty Library. Then all of a sudden the Events appear. Prior to those updates I don't recall this behavior. Rather, Events would appear almost immediately, though there was some Beachball while the app fully launched.
As we speak, I decided to go ahead and let iPhoto Library Manager have a crack at this. I definitely am not thrilled at the option of losing my Books and Slideshows, but since my original Library will be untouched, I'm willing to let it run and then compare the two Libraries.
I'll post further when I do...it's been running about 4 hours so far...
Dave