UPDATE: With iOS 7, Siri can now toggle system settings, including putting your iPhone, iPod touch, or iPad into Airplane mode. That would prevent your being able to track it in case of theft. While convenient, if you consider Siri access from the Lock screen a security risk, follow the steps in this article to disable it. (Note: iOS 7 screens look different, but the procedure is the same.)
If you've got a passcode set on your iPhone, Siri and Passbook will still work by default on your lock screen. This means that anyone who wants to talk to Siri or use one of your passes will still be able to. While it's convenient, it can be a bit unsafe. If you prefer having to use your passcode in order to access them, you can do so easily with a simple settings change.
That's it. You'll now be required to enter your passcode before using a Passbook or Siri feature from the lock screen.