Now that we're this deep in, I think opt-out should be managed via the wp-config.php file.

Decisions, not Options, is one of the WP credos, right?

/** Turn off Auto Upgrade **/
define('WP_AUTO_UPGRADE', false);

Done. That would keep it out of the hands of people who don't need to know any different.

(Otto, I actually notice the Chrome updates a lot more with Lion, which doesn't ever actually shut DOWN Chrome when you close it. So I have to remember to check for updates and have Chrome tell me 'Hey, you haven't restarted in a while....' which is an interesting 'bug' Chrome needs to address ;) )