Weng Chin points out something he describes as a security flaw in the iPhone 4S with Siri. In this video, Chin demonstrates that a user can make phone calls from the iPhone 4S even while it is protected by the iPhone:
At 1 minute and 5 seconds in the video, Chin shows using Siri to make a phone call despite the iPhone 4S being locked. This seems to be by design, however.
Having Siri enabled while your phone is locked allows for you to continue to have hands free usage of your phone, even when in a locked state. Apple does offer you the ability to turn Siri off on the Lock Screen, if you prefer more security over convenience. In previous iPhones without Siri, users could similarly dial arbitrary numbers using Voice Dial even when the iPhone was locked. Apple similarly offered an option for Voice Dial to be turned off on the Lock Screen only.