Another form factor option would be a Macbook Air with a double-sided display. Rather than flipping a screen around, you just close the lid to enter iPad mode, and open it for laptop mode.
Seamless app switching from iOS to OSX mode basically requires writing two different views, based on the same model, using a MVC design. That'll be easier for some apps (Safari, Calendar), difficult for some (Numbers, OmniGraffle), and maybe impossible for others (Xcode). It wouldn't make sense to try and force OSX and iOS versions of "everything" as some apps work best in one environment and not the other.
Windows 8 is half-doing this (from what I've read), and sounds clunky, but the verdict is still out on that.