Mark Rippetoe (videos) explains his position:
DL Alignment Part I:
DL Alignment Part Du:
Rippetoe DL Anatomy:
Thats a good point about how the mechanics of the sumo pull are different than conventional and that they should be looked at differently.
That being said, and I can only speak for sumo cuz my conventional technique is crap, it is possible for the bar to leave the ground while still in front of the shoulders, as long as the shins are there to stop it from swinging back. Thats the part where you increase your leverage. By rocking back in that position you can more easily break the bar from the floor, and then continue the pull from there, and very soon after breaking from the floor the bar will end up under the scapula.
But thats the idea behind shoulders back, is that your using your body as a lever to break the floor.
Like has been said, experiment, both techniques have merit.