The import of iPhoto Libraries changed considerably since Aperture 3.3 and iPhoto 9.3.
Now Aperture and iPhoto share a unified library format. Any iPhoto library (version 9.3 or later) is also an Aperture library. You can simply open your iPhoto library in Aperture and use it with Aperture or iPhoto as you like.
But you cannot any longer import single iPhoto events or albums; there is no longer an iPhoto Browser in Aperture. But you can import complete iPhoto libraries into an Aperture library, if you want to merge your libraries into one.
See these support articles:
When you are viewing your iPhoto Library in Aperture, you will see all your organization - the events will be projects, you will see all your faces, places,keywords, albums. It is easier to group projects/events in Aperture, so any new arranging would better be done after the transit.
Added: If you have any hidden photos in iPhoto, unhide them before you open the library in Aperture, for Aperture cannot unhide photos hidden in iPhoto.