there have been indications that Siri would eventually come to the iPad.
It really is hard to say without knowing the specifics of how siri works. If there is a lot of 'internal' processing going on, then maybe I understand. It might work on an iPhone4, but might be too glitchy to be up to apple standards. However, if it's almost all being shipped out to the NC server farm for processing, why does the A5 chip matter?
I think part of Apple's reasoning when it comes to the iPad is that that's not how they really want you to use your iPad (it's not like Apple are control freaks or anything). They view the iPad as mostly a content consumption device and the iPhone as mostly a personal tool that also plays games (remember when the iPhone first came out, Jobs indicated that he sort of hated the idea of it as a gaming device, but I guess the huge checks changed his mind). I think Siri will come to the iPad eventually, but probably not until it can do things like 'play the next episode of Arrested Development that I haven't watched on Netflix' and such.
As for now I think Apple believes people would just get frustrated by the fact that Siri doesn't play very well with third party apps, which are comparatively more important on the iPad than the iPhone, because of all the third party media content. It's not all that often that people are going ot use their iPads to schedule meetings and such. They probably want to wait for it to send to iPad when it's really ready for prime time, ie no longer beta.
That all being said, I think we're all kidding ourselves if we believe that at least PART of the exclusivity to 4S isn't a marketing ploy. Apple famously hates selling products on specs. If they were a garden variety PC maker, they'd have no problem selling the 4S as a massively spec'd up device and thinking people would buy based on that. Apple is playing psychologist, which they do very well, and realizing that while the 4S may be a huge upgrade internally, people won't go out and buy it unless they have something semi-tangible they can point to. So, I think it's several factors all adding together. Marketing + genuine usability issues + Apple defining how they view their iOS ecosystem (iPhone as a personal assistant and occasional gamer iPad as a content consumption device iPod touch as a mini content consumption device and gamer).
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how could you jailbreak to get Siri though, without straight up ripping off Apple's code? Apple has kind of sort of tolerated jailbreakers to this point, but if people were straight up ripping off Apple code to put in their phones, you'd start seeing Apple aggressively seek and destroy the jailbreaking community.