9to5Mac has done just that and they have ported it over. From the video, it appears to be very sluggish and the commands doesn’t seem to work at the moment. There are 2 possibilities, one being a processor limitation or perhaps the lack of drivers required for Siri to work as an integrated part of the iPhone 4 OS. While both are running iOS 5, Apple normally strips some functionality and features on older devices.
On actual usage, it doesn’t help right now since Apple servers are only authenticating Siri requests sent from iPhone 4S devices only. Technically, Siri should be able to work on previous iOS devices as it was originally a standalone app.
Since the iPad 2 has a dual core A5 processor, we wonder why didn’t Apple extend Siri for tablet users.
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