Preview at WWDC likely in June, followed by September launch.
Apple's 'Hey Siri' Feature in iOS 9 Uses Individualized Voice Recognition
Although unconfirmed by Apple, the discovery in iOS 9.1 suggests that Siri will be able to begin detecting specific user voices and determine whether or not the owner of the iPhone in question is speaking to her. Similar in vein to the way Apple aimed its Touch ID feature to work better and better the more you unlocked an iPhone using the fingerprint scanning sensor, it seems the set-up process will guide users into stating words or phrases to better acclimate Siri with each iPhone owner.
Found in General > Siri > Allow 'Hey Siri', the new always-on feature is the next step-up in the technology by Apple, allowing users to ask Siri questions or make changes within the iPhone's apps by simply stating "Hey Siri" near the iPhone. The new set-up process discovered today could also just be a way for Siri to work better detecting voices in general, and not be specific to each user. With the iPhone 6s and iPhone 6s Plus launching in just two weeks, it won't be long until everyone can find out for themselves.
Thanks Alan and Daniel!