I tried to update the Google Maps app that was pre-installed on my ICS 4.0.3 tablet, from the Market. I got "Package file was not signed correctly": There was no uninstall option for Maps. Since I was rooted, I used ROM Toolbox Lite to uninstall the app as root. Now, the Maps from the Market installed fine, and I even have an uninstall option. Ensure you backup the original Maps app apk before uninstalling it, in case you ever need the original/manufacturer pre-installed version.