Just to clarify, the pumpkins will not be planted in the grass, they will be in a bed heavily amended with compost. But I intend to let them sprawl out down the slope/grass-covered yard below the bed as they grow.

I like to grow C. moschata pumpkins (last year Dickinson Field) and they do a great job of dominating a weedy hillside. Outside the garden in a place I don't want to mow much, I make a big 3x8-foot compost heap with some chicken litter in it, and set five plants in the heap. I mow around the planting maybe twice, until the vines start to run. After that, the big leaves and vigorous vines definitely dominate over the nasty pasture fescue with no problem.
Last year I got over 100 pounds of good eating pumpkin from the five plants. One vine ran 20 feet, then up onto the roof of an out-building. They are great fun to watch!

WOW -- that's a lot of pumpkin!
C. moschata is the kind I can grow -- will look into Dickinson Field.

planatus -- HOW do you eat it? I'm always looking for good "pumpkin for dinner" recipes.

Melissa -- I agree with no need for landscape fabric -- just mow while you still can, and then let it go. It can be a little hard to get the mower through the taller than normal grass in the fall, but no big deal.