After trying all the usual things like rebooting, resetting the PRAM and saying 'Candyman' three times in front of a mirror, I sheepishly gave up and called Apple. It turned out that this has been happening to many people. Up until two days ago, Apple had been capturing reports to find the source. I was told that they stopped capturing reports when they numbered around 3,000.
What Apple found was a bug in the 8.0.3 updater, and apparently the bug could bite just about anytime since it didn't necessarily rear its head directly after the update. The bug, in various incarnations with various solutions, is to be cursorily found in the support forums, but here is the authorized Apple fix:
Hold down the option key as you boot iPhoto and you'll see a screen like the one displayed here. Just choose your library (there should only be one) and iPhoto will boot normally. Problem solved! As of now the problem hasn't resurfaced for me, and according to Apple tech support, it shouldn't. This bug will be squashed in a future iPhoto update but for now it's easy to get around.