DOF blur: not exactly easy to implement... but then neither are most of the ideas in this thread, right from the very beginning.
DOF blur is also not something that I'm particularly wanting to have, ever. It would almost surely come out as stupid as it did in Shift where you couldn't read your dash at all at 150mph, and if you switched it then you couldn't see the road. And in my opinion the whole switching idea is silly anyway. I can't imagine there ever being a situation where I can't easily see and read my dashboard and the road at the same time, though to be fair I've never driven faster than about 130mph and that was going straight. I may be focused more on one than the other, but the amount of difference in their clarity is infinitesimally small even when I'm quite busy driving and if even reproducable would be virtually undetectable on a TV or monitor screen(if it wasn't cartoonily exaggerated).
Amount of blur aside, the switch in focus when you move your eyes to something inside the car is so close to instantaneous that any attempt to re-create it would just result in an awkward clunky gimmick that would take valuable resources away from much more important things in both game development and system processing. And a perfectly good button on the conrtoller wasted. I know I'm not willing to give up my RA menu or headlight control for the "realism" of not being able to read my speedometer while driving unless I push a key. I'd much rather find a button to use for, say, toggling the HUD. Which might even be "easy" to implement.