Well, Apple's strength is looking at what's on the market and making something that both appeals to the masses and works really well..
They have a particular vision for how things should work, and they execute to that vision relentlessly. If it doesn't fit the vision, away it goes. If it doesn't work consistently and reliably, away it goes.
They would not release a touch screen MacBook Air unless they felt that it worked really really well. So far the answer seems to be "we don't think it would work" but I'm sure somewhere deep inside Apple is a lab where they are testing touchscreen-enabled laptops and maybe even iMacs.
Will it happen one day? Maybe. Microsoft is hedging on yes, hence the release of Surface Pro and so many other touch screen devices made by Lenovo, Samsung, Acer, Dell, Sony, HP, etc. Microsoft has had some successes and failures. Each of those companies is trying a different approach to answer the question "can my laptop be a tablet too?" Cynics would say they're just throwing different things at the wall to see which would stick. That might be a fair assessment. However, this is not the market Apple likes -- they want a sure bet with proven technology that they can run with.
OS X is not particularly touch-enabled. On the other hand, Mission Control and Launchpad look like they are designed for a touch interface. Maybe Apple is slowly working in features so that one day, when the technology and markets are ready, it will finally make sense.
It's possible that Apple already has a solution in mind. Windows 8 has set up the public perception that "there are touch screen computers out there, but some of them are pretty crappy." If you think about it, this is exactly the situation where Apple likes to step in with its solution. You could just picture Apple saying "Surface tries to do this but fails. So do all these other guys. Nobody has it right yet! Here's our solution. We think this will work."
We'll find out eventually...