Well, I suppose we're kind of splitting hairs to worry about that. In real use, the snowboard sinks into the snow to some depth, and the edge is often engaged in the snow <i>past</i> the widest points of the board - sometimes well up the nose. Check out the nose in these pictures, it's half in the snow!
http://photo.ruhtra.com/04311b/n311b16c.jpg
http://photo.ruhtra.com/04311b/n311b21c.jpg
However, yes, I would measure the running length of the board as the length of the board touching the ground, decambered. We have to establish a consistent reference, and I believe this is where manufacturers do it. I'll ask Sean, if he's not reading this.