The humor in some of the responses is a really nice touch; perhaps something I didn't expect from Apple, who is fairly straight-down-the-line with their dialogs and software.
Saying that, it's actually appreciated, and does a great job at attempting to establish an emotional tie with the user.
I also really like that the story proves you to be thoroughly correct about Apple trying to establish emotional ties. Here's hoping that they made that a part of their "corporate DNA".
Some of the humor and personality people detect in Siri may be grandfathered in from Wolfram Alpha, which has stock responses to certain queries. It's particularly good at joining in with movie quotes.
The Mac's desktop speech recognizer, which is so old most Mac users don't even know exists, even tells knock-knock jokes (in response to "tell me a joke" -- the script it follows is in ~/Library/Speech/Speakable Items). And this has been a feature of Mac OS since the pre-OSX days.
Saying that, it's actually appreciated, and does a great job at attempting to establish an emotional tie with the user.