I agree with @smartwatchr, taptic feedback is going to allow for instant communication with people close by, and the example above is the killer use-case.
Remember that you can already set custom haptic feedback for notifications for any person in iOS, including the ability to create your own vibration pattern and assign that to a contact. But this is different, because as @smartwachr pointed out, you won't have to open your phone to first text someone else. It's closed-loop haptic feedback, touch to touch.
But if the feature is half-baked, buried in the UI, or unreliable, it will be DOA. Adoption is the other side of the coin. A social service is only as good as the people who use it - just ask Google+. It's a feature that has great potential, but only if it's adopted en masse. Tablets took years of refinement, one has only to look at the Apple Newton or the years of failed windows tablet PC's to get a glimpse of tech that had great promise but wasn't mature.
Are people ready for this? Better yet, is Apple?