This is a useful hint for users who have their photos organized into folders and folders of events, all carefully named. This makes importing into iPhoto seemingly straightforward, especially with the advent of Events in iPhoto.
But neither dragging the folders in, nor using the File » Import to Library menu will retain your folder names (which in this case, are your event names).
I have found rather by fluke that hitting Undo after importing will restore your event names. It seems the auto-splitting function renames events to the date your images were taken, and undo just undoes that last bit.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080928040549638