I think they may eventually do this.
If you think about it, headphone jacks are now the limiting factor for the thickness (or 'thinness') of iPod shuffles and nanos. While these aren't Apple's main products anymore, what happens if the 3.5mm jack becomes the limiting factor for iPhones and iPads?
While this may be a few years away, I can definitely see Apple doing away with headphone jacks and moving to Bluetooth headphones.
Hopefully, however, this comes with huge improvements in battery technology!