I've watched several videos of Dave Tate explaining the sumo deadlift. He suggests getting your hips as close to the bar as you can, which typically requires a wider stance, and to squats your butt down with a more upright torso. You seem to be maintaining a similar ROM of the trunk as a normal deadlift. If you can squat down with an upright torso, the movement is much, much more like a back squat, putting very little torque on the lumbar spine. Would you be interested in a video or two of Dave Tate explaining it? I'm sure I could find it around here somewhere.
Here's an example video of a great sumo deadlift:
http://www.youtube.com/watch?v=ffB7M1OFb6M&feature=related