Siri may be your new best virtual friend, but heads up — she might be talking about you to strangers. A recently discovered security hole in Siri on the iPhone 4S allows anyone access to Siri even if your phone is passcode locked. Just by pressing and holding the home button, you can send text messages, emails, make and change calendar appointments, and get info on contacts.
Still, it's better to be safe than sorry. Follow these steps to secure Siri behind a passcode lock: go to Settings > General > Passcode Lock. Enter your passcode, then scroll down to where it says "Siri" and slide it into the "Off" position. This will limit Siri's functionality behind your passcode lock. Take note that this function is on by default, so change it right away if you don't want your texts and appointments falling into the wrong hands.