Any reason Google's Siri counterpart would be limited to new versions of the OS? Most (all?) of Google's core apps are available on older versions of the OS.
In fact I'm not sure I understand why Siri isn't available on the iPhone 4/iPad 2, etc.
Hint: When Siri came out, everyone with a Droid phone wanted it too. When the feature complete competitor comes out in full force, probably in Android 5? they will all be hilariously frustrated with how they can't use it because they have a 2 year old phone.