And from what they say in the source, these tests have gone well and the performance of the iPhone 4 could allow to install Siri without problems (it seems that the tests have already finished). Of course, it should also be mentioned that other people have achieved the same using unofficial methods. The same would not have happened with the iPod touch: its microphone is not good enough for Siri to pick up our voice well.
That said, there are two details: Will iPhone 4S sales hold up if the iPhone 4 is upgraded with Siri? The wizard is the main attraction of the new phone in the United States. And assuming that every iPhone 4 on the street right now gets the wizard… would Apple’s data centers handle all that new traffic volume that would mean? These are two of the surely many more factors that Cupertino will be considering right now. But the good news is that Siri may land on iPhone 4 soon, something that would make them more afraid of a competition that has already said that this Apple assistant means a threat to them.