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.
It's a way of bypassing the uncannny valley problem that plagues artificial intelligence and robot design: Avoid the uncanny valley by making things that don't try to cross in the first place. AI is going to be weird and break interactions in unexpected ways, so evoke something else, something that we expect to behave weirdly anyway.
Apple went one step further. Early reviewers are delightedly reporting that Siri says clever stuff when you say weird things. So she's funny, in addition to being kind of alien. Together, these combine to give her all kinds of room for error. She's more like a weird saucy artificial-person who sometimes doesn't understand you than a malfunctioning machine. She's Rosie the robot instead of HAL.
Thus armed, Siri is ready to face the slings and arrows of her demanding public. If I'm right about all this, the strategy is working. People are delighted with what Siri can do instead of frustrated by what she can't. Instead of egg freckles, we're getting "Back Bay. Find bridge clubs."