ok here's a very very rough and unpolished thought
you select "web document" and you get a dialog that says "do you want to do a mobile at site at the same time"
if you say yes you get two pages side by side, one for desktop size, one for the current "average" mobile width (not an easy choice but let's say there is one)
you design your desktop side and you drag and drop your essential mobile elements from the desktop to the mobile side
every new page created follows the same convention as above
when you're finished the preview gives you two views side by side
then when you export it automatically creates a js or php or htaccess solution to do a mobile detect with maybe a cookie to prevent looping
and the export puts the mobile in a subdirectory or a subdomain as required
its not perfect by any means
but it's a start and gets people thinking about accessibility for mobile devices
the mobile web market is exploding
those of us in web design are feeling the pressure
every single client this year has in one way or another asked me about this
having a solution to hand is better than not having one at all
those movie makers who said talkies would never take off lost everything
i do my best to design for end users but when it comes down to it i'm designing for the people who pay the bills