@Ipstenu - I'm glad someone brought this up because it's something I thought about when writing the post.

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

An option to choose whether to opt-in or not to automatic upgrades would never be added to core so whether I like it or not, they would likely have to be opt-out anyways. And of course, I suppose the wp-config way of doing it wouldn't hurt.