WordPress Automatic Upgrade is a plugin that automatically upgrades your wordpress version to the latest files provided by wordpress.org using the 5 steps provided in the wordpress upgrade instructions.
WPAU as I call it helps you easily upgrading you to easily upgrade your wordpress installation to the latest version provided by wordpress, without having to download or upload any files.
I just discovered it when I was having problems with my 2.5.1 installation – it’s linked from the WordPress.org instructions for upgrading. It’s also a good idea to check if you’ve been hacked . The older versions have a vulnerability that allows exposure of admin passwords and such.