This issue is resolved in Aperture 3.0.1.
To resolve the issue in Aperture 3.0:
- In the Finder, navigate to /Library/Application Support/ProApps.
- Move the "Aperture System ID" file to the Desktop.
- Open Aperture 3.
- Enter your Aperture 3 upgrade serial number when prompted to do so.
- Enter your previous Aperture version serial number when prompted to do so.
- Once your serial number has been accepted, you can delete the file that you moved to your Desktop (Aperture System ID).