Okay, so i'm new, and i've only just started reading WotC...so this might not be my place to comment on this thread yet (as it might be explained later on in the books) but here's what i've been thinking about Roland's world 'moving on' so far...

I think Mid-World used to be just like our world (with all its technologies, cars, planes, gas stations (i.e. Texaco!) etc). but I think that man/civilization, given the opportunity, will eventually destroy itself (through technology or whatever), and then it has to start from scratch again and build itself up. I think our world has had to do that throughout the history of man as well (think of all the lost/dead civilizations like the Aztecs or Egyptians - highly advanced at that time, but they didn't last and destroyed themselves and civilizations coming after (i.e. us eventually) had to rebuild itself).

It's like our planet after something like a natural disaster - like a volcano say - lava and ash destroys and kills all plant and animal life making the area barren. but eventually life comes back and starts to grow again and start over. so i think rolands world was like ours but then destroyed itself (though leaving bits and pieces - like oil tankers and computers and cars etc) though the people that live now don't know/remember what these things are for and do, and had to start from scratch, and evolved a different, more 'primitive' (i.e. feudal society-like)with different customs etc (but with magic!).

that's my thoughts (so far anyways)... i'll comment again (hopefully) once i've finished the series