This plugin works great. Here's my workflow:
1. Install
2. Upgrade
3. Get locked out of my blog
4. Delete the plugin via ftp
5. Upgraded blog now works
As long as you don't mind this process, you're golden.
Not sure why this plugin has such a great rating, but I guess it does the job. It cost me an hour or so to figure this out, though.
http://wordpress.org/extend/plugins/wordpress-automatic-upgrade/