Mac OS X desktop would run so slowly on an iPad. The A4 chip is slower than a 1GHz G4.
iOS is an optimised version of OS X for a mobile platform.
What could happen is that they put iOS into OS X (it's only 850MB) and use it in a touch mode like HP's Touchsmart.
The only reason to do that though is when you can hold a laptop like an iPad. This will be a difficult transition to make because of the ports and also because if they make it like the iPad, most of the apps expect the keyboard and mouse. They can tweak the standard UI to work better for touch but they will want the majority of apps to work best with touch.
They can create dual-mode apps that enter a touch-supporting fullscreen mode and make touch-support mandatory. This way all apps that have a fullscreen mode can work with touch. Then when this reaches a majority, the iPad/MBA convergence can happen.
For now, it will be the MB and MBA that converge, possibly as soon as next year and then we get 10.7 with features that push resolution independence and fullscreen support.
You can actually see what it looks like from the Youtube videos:http://www.youtube.com/watch?v=4eblWBqkjeo
The UI elements are too small in the normal state but blown up with RI, they should be fine