Even though we know a jailbroken iPhone 4 is perfectly capable of running Siri, we seriously doubt Apple will officially implement it.
TUAW reports that the iPhone 4S has a Siri-specific sensor in it. This one is totally distinct from the proximity sensor in all iPhones. That one detects when the phone is to your ear and turns off the screen — this one activates Siri if you use the “raise to speak” feature.
We have a hard time imagining Apple working up a software-only solution for something that currently relies on both software and hardware to function reliably.
Furthermore, Apple would much rather sell you a new phone for several hundred dollars instead of a Siri upgrade for $30.
Those points considered, we don’t think we’ll ever have Siri on our iPhone 4. But we really hope we’re wrong on this.