First off, I'm not necessarily against this idea, I'm still forming an opinion. However, I like control of my environments. My Mac lets me know when updates are available; for my apps, for the OS, iTunes, etc. My PC lets me know when Windows has updates. Hell, even Adobe lets me know when I have updates available. I think the current form WP takes of letting the Admin user know there is an update available is a familiar route, one most end-users --technical or not-- understand and are used to seeing in other software they own. Sure, Chrome and Firefox do it, but how many other software applications do?

We're not talking about the hosted version of WordPress, where .com has complete control over the platform, hosting environment, plugins, and themes. We're talking about self-hosted environments where the owner of the site has (and should have) control over what he/she does with it.

I would be in favor of an opt-in on the install screen; right under the "Allow my site to appear in search engines..." Why not an "Enable automatic updates" option? That way, it's "set it and forget it" while still allowing control. If a dev is setting the site up for a customer, they're making the call (or at least walking the customer through a technical decision). If a shared hosting provider has a one-click install (like GoDaddy and Mediatemple), they can enable it by default, and just have it be part of their terms of service. And if you're someone who manages your own hosting server, like myself and the last agency I worked for, you can make the call.

One other concern/question: the security implications. Being unfamiliar with how this would implemented, I'm curious as to the security implications of a site auto-anything-ing. Once the capability to auto-update is built into core, what's to stop a plugin developer --either maliciously or ineptly-- configuring his plugin to auto-update?