The Post Expirator plugin allows the user to set expiration dates for
both posts and pages. There is a configuration option page in the
plugins area that will allow you to separately control whether or not
posts/pages are either deleted or changed to draft status.

The plugin hooks into the wp-cron process and runs every hour.  It was designed and developed to work with WordPress MU, however it will not work in the mu-plugins folder due to how the plugin activation is setup to register the cron hooks.

It can be downloaded from the WordPress Plugin Directory – http://wordpress.org/extend/plugins/post-expirator/

Let me know if anyone encounters any issues and I hope you enjoy it!