That's my point - I expect it to get incrementally better between major releases[1]. That doesn't seem to happen - at least not on the response handling. Voice recognition seems to improve slightly, but that's the nature of having lots of training data.

[1] On new functionality that requires resources installed on the phone (the new sports scorecard things, for example), I understand that only happening on new releases. When I say "take me home" and it only starts navigation sometimes, it clearly has the ability to start nav based on something, so I expect that to happen more reliably.

I'm not positive it is.

If you are in Canada and ask Siri for something like "where's the nearest coffee shop", in iOS 5 you get "I don't support that in Canada", but in iOS 6 you get the expected results.

I'd assume the device version + iOS version + device ID is sent in the request and Apple maps this to the corresponding Siri version/database/API.. there would be no technical reason why iOS 5 can't see the same POI's as 6, only whatever business policy Apple has decided to implement. eg. freezing updates to the 5's database.

I'm sure the back-end processing is the same, but because 6 supports or will support new commands, then they are running separate instances & databases which results in varying update schedules.

As long as they don't restrict based on device model (only iOS version) then I'm happy with that.

They said it doesn't improve between releases. As opposed from from release to release.