Opt-out please. I like Ipstenu's comment to use wp-config.php to determine that and ask whether to turn off updates when installing WordPress, just like with the "block search engines" bit. As a developer I'd love to use the latest features of WordPress and not have to worry about back-compatibility, so knowing that the majority is upgrading with every new WordPress release I'll be able to use functions like submit_button without having to worry about users running versions prior to 3.1.

So yes, I want automatic upgrades and I want them on by default. I'll switch them off myself for larger projects where I carry out upgrades manually locally then test them on a staging server and finally deploy to production. So having an extra constant in my wp-config.php is nothing compared to what I already have to do to deploy :)

~ Konstantin