Currently in 5.6.2.1, if you turn on full page caching and use a mobile theme, mobile users could see the desktop version of the site and desktop users could see the mobile version, depending one which version was cached. Turning off full page caching usually increases page load time so dramatically that it is unacceptable. So basically mobile themes and altering output based on devices are severely broken.

If there were a separate cache for mobile in 5.7 this issue could be fixed. Perhaps providing functionality for a separate cache for tablets or TVs or any arbitrary breakpoint would desireable as well.

I think you would just need to change the dispatcher to check for mobile prior to loading the cache and also building the caching mechanism to store the pages in a separate mobile-cache folder if the request is from mobile. I just can't figure out the caching mechanism thoroughly enough to alter the cache path to create a viable fork.

Any thoughts on a separate mobile cache?