Under certain circumstances, the Aperture 1.1 Update won't install on Aperture 1.0.1 as intended. When this happens, Software Update won't offer you Aperture 1.1, and/or the standalone Aperture 1.1 updater says that no eligible previous version was found.
To resolve this issue, simply reapply the Aperture 1.0.1 update before using 1.1 again. You can get Aperture 1.0.1 using Software Update or Apple Downloads.