Siri doesn't sound human -- and that's why you'll forgive her when she doesn't understand
I'm curious about whether this is a technical limitation or a design decision. I know that it's possible to do voices better than this. I've been on hold with Apple's call-in system and they've managed to make the machine reading me back my phone number sound like a human, with the right rhythms and cadences. But of course the call-in system has a much smaller script of things it needs to say, and it need not live on an iPhone, so maybe a system that complex is not possible for Siri.
But I think that it may have been a choice on the part of the designers. She sounds like an older robot, which means you don't expect her to be as smart as a person. When she doesn't understand you, you assume that the fault was at least partially with you and with how you commanded her (it). (I keep thinking about BERG's be as smart as a puppy exhortation and the need to give robots losing personalities.) Apple made some stunning promises in the ads and demos (while the small print reminded us that Siri is in beta). Now that Siri is out in the wild, there are going to be all kinds of strange interactions. The inhuman voice softens the blow of things not working, by lowering our expectations of what will work.