I’ve successfully used the ThreeWP Broadcast plugin to do this on a WPMU installation.
The basic concept is that you have one ‘broadcasting’ site from which you can broadcast content to other sites. The content can be posts, pages, custom post types, custom taxonomies, post meta or attachments.
If content is updated on the ‘broadcasting’ site, the updates will be reflected on the other sites. More info can be found on the ThreeWP Broadcast WP Plugin Directory entry linked above.