I understand the value in theory, but I just can't imagine any graceful, user-friendly execution of this. It's a proprietary connection type, so it's not like it's going to be the de facto industry standard at some point down the line (a la the optical drive phase out comparison). And it wouldn't be a 1:1 replacement of an existing standard (a la changing the 30-pin to lightning), since it would have to perform multiple functions. This creates a number of problems.
To use any existing wired headphone purchased since time immemorial, you will need an adapter. This will most likely be purchased separately, will not be inexpensive, and will be very easy to lose. Every time you unplug or switch the headphones to another device, you'll have to store the adapter somewhere it won't get lost or leave it connected awkwardly to the phone or to the headphones. This creates a pretty clunky user experience. And unless there is some sort of daisy chaining functionality, listening while plugged in will no longer be an option on any wired headphones.
That said, I could imagine Apple including a pair of newly-designed earbuds that have a lightning port, allowing them to be connected wired or wirelessly, and to be charged by the phone. That would be a rather elegant use of the technology, and could make wireless headphones a lot more convenient by eliminating the need to charge. But unless they open the standard and make lightning non-proprietary, I don't see how it can create anything but headaches for every existing and future non-apple audio accessory.
