I believe the popular notions going around is that they want a controlled release to Siri. So, that means testing on one device. Secondly, I don't think that the iPad 2 has the noise cancellation mic, and that that affects the language recognition capability. Who knows for sure, though.
As far as API is concerned, I guarantee that we'll see Siri hooks sometime. I'd guess that it would be like how devs can register apps as handling file types. When you open a mail with a PDF or DOCX in it, the user gets to pick what app opens it, but it's not every app. It's only apps that can actually service the doctype (as defined by the developer).
Can anyone confirm that placing the iPhone up to your ear automatically engages Siri if nothing else is happening? I think that I read it in one of the reviews last night, but now I can't find it.