Siri's UI intends to mimic a person that understands what you are saying. In practice, it gets it wrong in hilarious and frustrating ways, breaking the illusion.
So "uncanny valley" seems ok to me.
E.g. something "50% real" is so far off, we psychologically dismiss it as a cartoon, drawing, whatever. It's not trying to compete with real.
Something "98% real" freaks us out though. Surprised I have not yet seen this link:
http://blog.codinghorror.com/avoiding-the-uncanny-valley-of-...
We are OK with stupid computers, we don't expect anything from them - we order them around is very formal language.
But now there is an attempt to use natural language - and it doesn't work right. So it's actually better not to use natural language, and just stick to the formal language.
That's where the valley part comes in.
You know when your GPS says "recalculating" in a condescending voice? That's the uncanny valley of text-to-speech.