I had been thinking about something along these lines as well. Not quite the same as the OP, but a cross between that and the Asus Transformer.
Macaddict16
This still does not solve the fundamental problem that you end up with a device that compromises what's great about the Macbook Air and what's great about the iPad.
I assume the argument for such a device is that the merger of iOS and OS X is inevitable and that people only want to carry one device. I think there are three points that Apple has made the last decade that demonstrates that those in Cupertino do not believe this is part of their "grand narrative."
1.
Devices and their components should do what they naturally do. This point was made when introducing the iMac G4 and why Apple didn't just chop the back 1/2 off an original iMac. It was reiterated on why Apple does not have multitouch notebook displays.
2.
Ascendency of iCloud as demotion of the Mac. With an api that allows app developers who are on both OS X and iOS to sync documents and whatnot, I imagine if you asked senior execs at Apple for such a compromised device, they would say having everything on one device is no longer necessary. It is telling that a slogan such as "Mac as a Digital Hub" was only killed in 2011. Everything Apple does for the next decade should be viewed through the iCloud prism.
3.
iOS allows OS X to be OS X and OS X allows iOS to be iOS The two different operating systems allows Apple to get away with compromises that each have to make, but will share certain features such as gestures, flash-based storage, and digital distribution.
24 months from now I imagine there will be a lineup of devices subordinated to iCloud that will remain relatively stable for a number of years. Every device will have primarily flash-based storage, lack optical drives, and be inherently thin and light. Those with keyboards and trackpads will run OS X. Those with multitouch displays will run iOS.