I have a pack in my install.xml containing file elements with the override attribute set to false. My understanding was that after the first install, the file would never be overridden for subsequent installs. When I run the installer, files are overridden as if the attribute was set to the default value update. I was able to verify this by manually changing the modified date on a single file and running the installer. The file that was changed was the only one that was overridden in the pack.