Patently Apple:

In the future, if Siri doesn’t recognize the voice of the owner of the device, accessing Siri or the computer associated with it will be impossible.

And:

Today a user will say “hey Siri” and Siri will respond. In the future, the command to call up Siri may be customized to your voice. For instance, a user sets up Siri to recognize the phrase “Hey there, Boss.” The customized phrase and the voice must match what’s in Siri’s database before the digital assistant will respond. The customized phrase is technically referred to in Apple’s patent filings as a “Lexical Trigger.”

And:

In the future, in order to access Siri, a person will have to know a passcode, use a fingerprint or be recognized by the system via face recognition. Unless your iDevice recognizes you on multiple security levels, Siri will remain unresponsive to commands or requests.

As long as I can turn this off via a setting, this seems like a natural evolution.