Siri is definitely the most sought-after and talked about feature of Apple's new iPhone 4S smartphone, the virtual voice-assistant can make calls from a locked screen also when a pass-code is required to unlock the device. This is no security-flaw but is by design and can be disabled as demoed next.

Since, this is "by design" and no security flaw. You can disable access to Siri from the passcode lock screen by navigating to Settings- > General -> Passcode Lock -> Disabling "Siri" toggle (which is to disallow access to Siri when locked with a passcode).

Disabling Siri From Lock Screen