I don’t think it’d be fast enough. Even in damn-near-perfect signal conditions I have a RTT over 100 ms when I’m on AT&T 3G. That and how lame would it be if Siri ceased functioning entirely (not just the maps/other network-requiring bits) when you had no coverage?
Given that the original product that Apple bought worked that way, I'd find it hard to believe that the newly integrated one changes things that significantly. If they did get it working locally it would be very interesting.
