However, as I understand it, iPhoto and Aperture use a single, unified library and any changes done in one program sould be reflected in the other.
No, that is not what is meant by "unified library". It means that iPhoto libraries and Aperture libraries are compatible. You can open iPhoto Libraries in Aperture and Aperture Libraries in iPhoto, and any changes will be visible in the other program, if you open the library with the other program. Both Applications can use the same libraries.
But if you have a separate Aperture library and a separate iPhoto library, any changes you do in Aperture using your Aperture libray will only apply to the Aperture library. To see them in iPhoto you need to open your Aperture Library in iPhoto and vice versa.
Thanks for your reply. I didn't realise there were still two separate libraries, one for iPhoto and one for Aperture. I went ahead and deleted my Aperture library (since it was just a copy of the iPhoto one), then opened Aperture and selected "Switch to library" and selected my iPhoto library. Now it is working fine, where changes are properly reflected using either program. FYI for anyone else reading this, from the Apple notes it says you cannot have both iPhoth and Aperture open at the same time when they are accessing the same library.