The default settings allow users to activate Siri at any time, even if the iPhone 4S is locked. While this could be a nice feature if you’re the one picking up your iPhone, others could use this as a way to activate Siri and learn about your upcoming calendar appointments, send emails or text messages, or reveal information (such as addresses) about you or your contacts.
While this is limited, as Siri can’t search the web or launch Mobile Safari while your phone is passcode locked, it can still pose a significant risk in revealing potentially sensitive information that you have on your device.
Fortunately, this can be disabled by switching Siri to “Off” in your passcode lock settings, but this will leave you unable to use Siri at all if your iPhone is locked with a passcode. Hopefully Apple will fix this soon, but in the meantime, the choice is between convenience and security.
*Update: It seems as though Siri will even allow you to make calls from your passcode-locked iPhone 4S.