As Clos90 said, the devs have stated that they won't do it, mostly because dynamic map changes, as well as the sheer dead weight of work involved.
That said, when the devs did a poll a little while back, pretty much the most asked for thing was a pre Doom start, so who knows.
I agree that it would be interesting - the Starks had only just achieved total pre-eminence in the North, the Lannisters and Gardners did not yet have anywhere near the totality of the Reach or Westerlands, the Riverlands were in total chaos, the Stormlands only marginally better - as well as the Durrandons being exhausted from putting down the pretensions of the Masseys to being independent kings, Dorne being almost as disarrayed as the Riverlands, and the Vale of Arryn being only marginally better. And of course, the giant horror that was the Freehold nearing its height. Interesting, and most likely fun, and I would really like to play Theon Stark the Hungry Wolf and raze Andalos to the ground, but it would require a massive amount of work - lots of events to spawn invading Andal bands, as well as some way to simulate the near universal survival of the Great Houses, just to name a few.
