It doesn't even need to be that complicated. A Wordpress install is already capable of checking for new versions and updating itself. All that's needed is for this to be checked and performed at various intervals using a wp-cron style scheduler (e.g triggered by user visits).

Is there any other CMS that do that? That would be a nice way to update. I remember reading somewhere a guy had a script that checks for new version everyday and upgrades automatically.

For me, I am subscribed to their blog so get notified by e-mail when there is an upgrade, from there it takes less than a minutes to login to my admin and upgrade. As a matter of fact, I did my last update from my iphone when I was on the road.