Oh come on, the iPhone 4 can run Siri. The 3GS can probably run it. What the heck is it doing that it absolutely requires the processing power of dual cores? Floating point mathematics? Video encoding? I don't think so. Sure you have the voice recognition and the algorithms associated with that, but the 4's processor is no slouch, and it has no problem with 3D gfx processing which requires huge amounts of complex math. Siri outsources some of the work to Apple's servers anyway.
I'm reasonably confident Siri will run on the 4.
I'm reasonably confident Siri will run on the 4.