To install Zimbra on a debian 5.0 Squeeze you'll have to modify those files:

 get_plat_tag.sh 

        fi
        grep "5.0" /etc/debian_version > /dev/null 2>&1
        if [ $? = 0 ]; then
            echo "DEBIAN4.0${i}"
            exit 0

 and the debian version:

5.0/squeeze/sid