Those of you using Siri on your iPhone, there's a default setting that lets anyone bypass your lock passcode and use Siri. This is allowed by default (bad Apple! No security biscuit!). You can turn this off, but you have to go into Settings for the Passcode lock:
You want to change the highlighted setting ("Allow access to Siri when locked with a passcode") to OFF.
The reason that Apple made this the default is so that you could use Siri to get directions when you were driving down the road. This is useful in increasing safety because drivers don't have to fumble entering their passcode, but it leaves a pretty big security hole when they're not.
So pick your poison, but at least you're making an informed choice.