Since the launch of the iPhone 4S and the introduction of Siri, I have read many accusations that Apple has restricted Siri to the new model to help boost sales. Considering that much of the heavy lifting—like speech processing—is handled in the cloud, and the fact that Siri has been successfully ported to the iPhone 4, the assumed A5 hardware requirement started to fizzle. According to an article on AllThingsD, Siri’s iPhone 4S exclusivity really does look like a selling point:

Sterne Agee analyst Shaw Wu says his latest checks with industry and supply chain sources show broad sales strength across Apple’s entire iPhone portfolio, but most of all for the 4S. Evidently lots of folks who could be spending $99 on the iPhone 4 are opting to fork over another $100 for the 4S — and a lot of them are doing it for Siri.

“Despite global macroeconomic headwinds, Apple continues to defy conventional wisdom with a higher-end product mix,” Wu says. “Talking to industry sources, what’s driving the 4S is better than expected reception of its new Siri software.”

It may also be possible that Apple plans to roll out Siri to the iPhone 4 at a later date. For the initial rollout, Apple may have labelled Siri “Beta” and restricted its user base size solely to ensure their infrastructure could handle the demand. Once Apple establishes confidence in the service, they may very well open up Siri for the iPhone 4 and move it from beta to gold master.