Hey guys, I had the same problem, I accidentally set the wrong year on my new Nikon D5100, and all of the photos i took were dated last year, didn't notice it until i had imported about 100 photos into iPhoto, and I couldn't figure out why they weren't showing up at the bottom of the event list.
So, like many of you guys have done, I changed the dates on the photographs, ticking the change original files, and it worked for the individual photos, but did not change the event date. So I have been mulling about this problem for a few days now, and I just found this forum post via google.
After reading this, I decided to try Merging the Events (event grouping was done by days) into one event, and then Auto-Splitting the Event that was just merged, and it corrected all of the Event Dates and put them at the bottom of the event listing sorted chronologically. It will probably delete any custom names you have for events, I wouldn't know because I just left the event names as the date the event was made.