I fixed this issue and was able to access the admin panel. I documented it here…
If this is related to WPAU or WordPress Automatic Upgrade plugin conflicting with the automatic upgrade function of the WP core, you can disable WPAU and try upgrading again:
http://kramfs.com/2009/02/11/wordpress-automatic-upgrade-fatal-error-cannot-redeclare-pclziputilpathreduction/