I don't have a garden yet, but I do have the new SFG book from the library....
He says to put 1 watermelon or 1 pumpkin per 2 square feet (cantaloupe and smaller melons only need 1), and train them up a trellis. He swears that if you start with a trellis in place and help train the vine as it grows, it will grow itself strong enough to support however many melons/pumpkins would normally grown on the plant.
Having never done this or seen it, I don't know how true that is, but he says 1 plant per square foot... I'm going to give it a try when I get my bed built!
He also says 1 plant per 2 square feet for winter and summer squash.