Well, I've used it as mulch in the garden...moderately effective against weeds, so I suppose it's the same thing. Becomes less aesthetically pleasing as time passes, however.

Hocasati,
I would rake up the grass clippings and set aside in a compost pile. They will be good for mulch in your beds later. To kill the grass, try laying down a sheet of black plastic. Weight down the edges so it wont blow away. Very effective and quick. Get it at any hardware store.
Best of luck.
Dan

I made a couple of garden beds where I had lawn, by spreading disassembled cardboard boxes on the lawn, wetting them down, then placing a layer of compost then mulch on top of them (you could use your grass clippings as mulch). I waited a month then dug holes and put plants in.Pretty much an instant garden bed and a great no dig garden.The worms love it too.Its been many months now and the plants are very happy. I just give them an occasional dressing of compost and mulch.

Get the book "Lasagna Gardening" from your library or buy it for your personal library. Lots of great ideas for sheet composting large areas. The grass will come in handy for the sheet compost, but it needs other ingredients to acheive your goal for the back yard.