The reason OS X has a different interface than iOS isnt because one came after the other or because this ones old and this ones new, Federighi said. Instead, its because using a mouse and keyboard just isnt the same as tapping with your finger. This device, Federighi said, pointing at a MacBook Air screen, has been honed over 30 years to be optimal for keyboards and mice. Schiller and Federighi both made clear that Apple believes that competitors who try to attach a touchscreen to a PC or a clamshell keyboard onto a tablet are barking up the wrong tree."
i had this in my notes from a while ago, it seems like apple would be reluctant to do a combo device because of the UI differences.
interesting thing though, the science museum in london has OS X devices with touch screens in some of their exhibits. they seem to be just running a keynote presentation or something but i was there early one morning and one failed and just had the OS X home screen working perfectly well with a touchscreen (assumed 3rd party enabled)