Do Local Security Policy settings, specifically those under User Rights Assignment, replace or append the existing settings when applied through a Group Policy Object? What about the settings applied through other GPO's?
Settings that are defined in domain GPOs override settings on the local machine. Settings that are undefined in the domain GPO will remain with the settings in the local policy.
A great way to test is to use Resultant Set of Policy. It will tell you where and through what means settings are applied.
Something to thnk about if using Vista SP1 or above