What I meant is that the page itself doesn't tell you anything about how you would actually get it.

The first thing I did after clicking on the link was search the Play Store for "Google Now" - obviously it came up a blank.

What this means is that I'll probably forget all about it by the time I either:

a) get an Android 5 phone/tablet, or b) update my Galaxy S2 or Galaxy Tab to Android 5

Google could have alleviated some of this by having an app ready for download and linking to it. I have to admit that I hoped that this was the way forward since they released Chrome Beta, as opposed to relying on manufacturers/carriers updating their phones. Alas, it wasn't to be.

Technically speaking it is Android 4.1, but yes I get your point.

Anyway. You can tell from the way this is implemented that this is a core OS feature, not an addon app stacked on on top. The reason it's not out for ICS or older versions of Android is because they can't run it. They lack the pieces it uses through out.

I think the reason it's not an APK is due to the fact that it's baked in directly into Android 4.1's search.

Actually it's also one of the quick actions on the lockscreen, along with camera and unlock. That should help discovery.