Welcome to iPhone 101, the series that explains the basics of iPhone operation. This time, it's making sure Siri doesn't let anyone use your locked iPhone.
Almost immediately after the release of the iPhone 4S, TUAW started receiving emails from readers who noticed that even when they had a passcode set on the lock screen, someone could pick up their device and issue commands to Siri. This means that unauthorized persons can easily pick up the iPhone 4S, press and hold the Home button and converse with Siri. Fortunately, there's a way to disable Siri while using a lock screen passcode.
The Sophos Naked Security blog noted that those unauthorized users can do everything from writing an email or sending a text message to maliciously changing calendar appointments. Blogger Graham Cluely notes that it's easy to disable Siri while there's a passcode in effect, and wonders why Apple didn't set the iPhone 4S up that way by default.
To make sure Siri is deaf to commands when there's a passcode on the iPhone 4S, enter Settings > General > Passcode Lock, and slide the Siri option to Off. Now, when your friends try to make a prank call to your girlfriend using your iPhone 4S, they'll find that Siri is unwilling to be a participant in the prank.