
Siri was almost immediately ported to the iPhone 4 after the 4S's release by @stroughtonsmith. Nine days later Siri is up and working on the iPad.
Kind of.
Developer @stroughtonsmith was able to get Siri working smoothly on the iPhone 4 and has been privately working with other developers such as @jackoplane to make further progress. As of right now progress is still slow in regards to getting Apple's servers to authenticate Siri's requests. Since most of Siri's functionality relies on Apple's servers to do the heavy processing the personal assistant, as of now Siri on the iPhone 4 is nothing more than a functioning skeleton.
Despite @jackoplane's and the jailbreak community's slow progress in developing a way around Apple's server restrictions, @jackoplane successfully ported Siri to the iPad 1. Our own Joshua Tucker (@joshmtucker) vouched for @jackoplane and the legitimacy of his claims.

Siri running on the iPad 1 confirms our earlier suspicions that Apple's claim Siri can only run on the iPhone 4S is pure marketing hogwash. Apple would only need to change the authentication process used by its servers, sell Siri in the App Store, and reap the rewards. However, Apple has maintained that Siri is still in beta, albeit publicly. Perhaps when Siri out of Beta Apple will expand its availability.
Source: @stroughtonsmith [via Jailbreakstory]
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