Does this happen with all libraries? If you only have one library try making a new empty one and add some images into for testing.
If it happens in the new library try making a new user and running the same test.
This will help to identify if the problem is with your account (works ok as new user) or system wide (also fails with new user).
Also in cases of UI mess ups a good place to start is by removing your Aperture preference file (see Aperture 3: Troubleshooting Basics for a description of how to move out the preference file).
Thanks for the response! Unfortunately, I don't have any of these answers right now, but I will give them all a try and post a follow-up once I get a chance. I have only done a serious amount of work with one library, but I'll try making a new one and see if it happens there, and also as a different user on my system.