I've started using grass clippings as mulch, but a couple weeks after planting. It seems to be going well so far. I've been following the guidelines from this page:
http://www.ext.colostate.edu/pubs/garden/07007.html
I'll add a layer after each mowing until I have several inches. That should be a good nitrogen feeder and keep some weeds out. I probably won't need the newspaper or cardboard (crossing my fingers).