That sounds bizarre to do with products in the wild. With chip yield being so fickle at such high concentrations, is having a core being disabled for most of the products' lifestyle a bad idea to activate suddenly, in case one core wasn't functioning properly?

If Apple plans on enabling the second core with a software update then they would have tested both cores during manufacturing.

Disabling hardware and then enabling it through software later to pretend that it's an update isn't how Apple operates and would be a really weird thing for them to do. The only thing I can think of that even comes close is when some Macs shipped with really early draft 802.11n hardware when the software wasn't ready, so they only supported 802.11g until a software update was made later. However, that was a case of the software and spec actually not being ready yet, not simply a matter of disabling something that would otherwise work fine just to have something to brag about later.

It wouldn't be unprecedented for them to release an update which enables features which were always in a chip but were unusable before, although it's not an identical situation:

http://blog.macsales.com/9102-secret-firmware-lets-late-08-m...

The fact that this feature of that firmware update was so hidden that it was only noticed when people saw different behaviors makes it rather the opposite of what's proposed. Additionally, I seriously doubt that Apple deliberately withheld the ability to use 8GB of RAM in software only to add it back later. Much more likely it was a bug or just something that they didn't have time to finish. Neither would apply to running a single core of a dual-core processor in an Apple TV.

How about Bluetooth on the second generation iPod touch? That's a prime example of "chipset supported this, Apple locked it out with software and unlocked it with an update."

I wouldn't really call it "locking out". The Bluetooth stack in iOS was extremely incomplete before that update. If Apple had offered it at release, it would have only supported the Headset profile which is near useless on a touch. At the time Apple genuinely believed that with their accounting scheme, they couldn't give away hardware features without charging. To strike a balance between those concerns, we got Bluetooth in the iPhone OS 2 update for a fee.

Either way, that isn't what is happening here. This is just CPU manufacturing 101. New chip, new processes, high bin rate. You mitigate this as much as possible by disabling features on binned chips to make them sellable. Intel has done it a number of times, typically when making a big change like the introduction of the Core Duo.