It happens everywhere. 90 percent of mexican voice-over work is done with the "Mexico City" accent, which has been determined to be the least reigional dialect and therefore the easiest to broadcast everywhere. I've heard mexican anime fans wail and gnash heartily over this fact. I imagine The Beverly Hillbilliies would be very dull when dubbed with all cast members having the same accent.
Wasn't there someone here with an accent story regarding call centers (centres) and Canada?
And as I've said before, though I grew up in Alabama and spent my first 25 years there, I never had a very pronounced southern accent and eliminated it completely from my speech by age 20 or so. Part of the reason came to me when I listened to a lecture by a brilliant architect with really groundbreaking ideas, but didn't take him seriously for the first half of it because he had a thick Kentucky accent and was nicknamed Sambo.
Reigional accents, like regional delicacies, customs, music, and architecture, require insular communities to foster them. A mere century ago, people rarely traveled more than twenty miles in their lifetime. Physical and class barriers tended to reinforce accents. Even the large cities had, by necessity, subdivided into smaller self-sufficent communities with regional accents of their own. Bronx accents still exist, and they're different from Long Island or Manhattan.
Cars, relocation, and mass media all break down accents. Our children are exposed to nationally-televised voices (with that 'generic' midwest accent) just as much as their friends from around the neighborhood, if they're lucky enough to have any. More and more often we have to drive to wherever our kids can play for as long as we have 'free' time, then back in the car with its national radio show and DVD player.
My oldest sister, 11 years my senior, still has some very Alabama pronunciations, especially in O sounds. Her husband does too, though neither of them match their parents. I started with far less of an accent than she had and have none now. Her children, aged five to fourteen, have very little southern accent in their speech at all.
I'm not sure how I feel, though I guess I can't really lament the loss of something when I actively stamped it out at home, so to speak.