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
Please do post those videos. I have found that whenever I try to have a more upright torso my hips come down which causes them to rise faster than the shoulders on the way up. The only way to counter this seems to be to tighten up the hamstrings which causes the hips to start from a higher level making for a more horizontal back.