As per the latest rumor from JailBreakNation which was then confirmed by AppAdvice claims that Apple is testing Siri on devices other than iPhone 4S. It means that company has plans to bring the artificial intelligence through a software update in near future. According to the sources, specifically, Siri has been tested on iPhone 4. Though there is no reason that it would not have been tested on others.
However, as per AddAdvice sources, it was installed on Apple employees devices while in the beta testing. But removed due the fear of leaks. It shows that it would become a reality if the testing has been successful. Some other sources said that Siri integration on devices other than iPhone 4s has worked really well except iPod touch 4G where the microphone is not good as compared to iPhone 4.
Through these sources, it can be concluded that Apple would definitely implement Siri to other device if it worked perfectly during beta testing. There is no doubt that the company would not want to minimize the user experience. Hence, devices where the feature has not been that efficient, the support would not be provided in any case.
Anyways, hackers have also ported Siri on iPod touch 4G after spoofing Apple servers with a certain line of code and changes in system files. Otherwise, Siri GUI has already been installed on iPhone 4 and iPad 2 from sometime now. Before the hack is made public, they are waiting for iPhone 4S JailBreak which would come out soon.