For those late to this discussion that didn't bother to read any of the 7 pages of posts:
Apple purchased Siri which was an already existing product that was on a par or just behind other voice control apps.
At the same time Apple purchased Audience which designed Voice Recognition/Noise Reduction chipsets.
Apple paired the two technologies for their much improved version of Siri. The Audience chipset works very well in suppressing background noise and heightened voice recognition. I've dictated a memo on my new iPad in a crowded coffee shop.
The only iDevices which have the Audience voice recognition hardware are the iPhone 4s, the new iPad, the iPhone 5 and the 5th Generation iPod Touch. These, not by coincidence, are the only iDevices to have Siri.