I came across some interesting videos featuring Helen Atthowe in Montana. She uses living mulch between rows of vegetables, and mows the mulch periodically, but does not weed. At one point she mentions that mallow -- usually considered a weed -- turns out to be a good nitrogen scavenger. When it's cut back, it adds more N than legumes or hay, about 80 pounds N per ton. Here's one of the videos:
http://www.youtube.com/watch?v=uJk4R1xpMC8&feature
She also mentioned that because of the living mulch, she gets earlier peppers and tomatoes than anyone else at the farmers market, and the plants are healthier and more vigorous.