One reader tipped us off that the iPhone's emergency call feature can be used to call any phone number and not just designated "emergency" numbers such as 911.
The emergency call feature is available even when your iPhone is passcode locked, and raised this reader's concern that your mobile minutes could be used without your knowledge.
In searching the internet, it appears that this issue has been around since at least firmware 2.0.2, though there appears to be some debate about whether or not it is an intentional feature.
If your iPhone is actually stolen, you will presumably discontinue service and that would prevent ongoing usage of your minutes. But it still leaves opens up the possibility of intermittent unauthorized usage.