Siri’s smart. Smart enough to bypass your lockcode. Yup, it’s just being realized by hundreds of 4S owners, but you can actually call and do other things from the lockscreen while Siri’s locked. That’s a problem when people get hold of your smartphone. Who knows what information they’ll extract from your iPhone.
Anyway, to avoid problems with this, you should change your settings. Go into passcode settings and turn Siri off from the settings. I sure wouldn’t like people snooping around my iPhone.