I was under the assumption that Siri needed the 4S's DSP capabilities on the new processor.<p>It seems that it can be put on iPhone 4 as well
(<a href="http://venturebeat.com/2011/10/15/siri-iphone-apple/" rel="nofollow">http://venturebeat.com/2011/10/15/siri-iphone-apple/</a>)
It could be that Siri requires the extra processing power of the A5 chip. If this is true, though, I don't understand why Siri isn't available on the iPad 2. It's silly to not give it to iPad users solely to drive 4S sales.
If I remember correctly John Gruber theorized that the reason for offering siri on the 4S only was just a way to keep the initial number of users down while they are still testing it.