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Keep in mind Siri is still Beta, we are nowhere near seeing where Apple will ultimately go with it...or "her".
There was an article about this recently on GigaOM.
It's inevitable that Siri will be implimented widely, in my opinion. Older iPhones have been hacked and been able to use Siri passably well, so I would expect to see backward compatability pretty soon. First off, people that don't have Siri, want to have Siri. That is reason enough to expect to see it across platforms, and it gives Apple something to wow and amaze the masses (including me). Earlier this week, I sat around with 3 other reasonably tech savvy individuals, one of whom had Siri on his iPhone, and spent at least an hour playing around, asking questions, etc. That is something I haven't seen since the iPad was released.