Siri: iPhone 4S not really locked when the phone is locked

An iPhone 4S glitch allows access to the Siri voice-activated virtual assistant when the phone is locked, adding another item to the list of problems plaguing the device.
A default setting lets anyone who picks up the iPhone 4S to send emails and texts or access a user’s address book, calendar and personal information simply by speaking into it and activating Siri without a passcode, even when the phone itself is password-protected.

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http://www.forbes.com/sites/mobiledia/2011/10/24/siri-security-glitch-adds-to-iphone-4s-problems/

To turn off Siri access when the 4S is locked:
Settings ->general -> Passcode lock -> Toggle the switch for
“allow access to Siri when locked with a passcode” to the off position.
After this action is taken, the handset requires passcode entry before Siri can be used.