jnicholes/Jared,
I would not worry about the toxicity of the grass itself as dogs instinctively know which types to consume. What I would more worry about is if you have added any chemicals to the lawn; the chemicals - fertilizer/pesticides/herbicides, would definitely be bad for dogs.
My dogs have a favorite patch of rye grass. They also eat goldenrod leaves and the leaves of the Helianthus angustifolius aka Swamp Sunflower which they love to eat. Neither of my dogs vomits after snacking in the yard.
If you want to be sure of a safe area for your dog to snack, I suggest you set aside a part of the yard and plant intermediate wheat grass/Thinopyrum intermedium, perennaial ryegrass/Lolium perenne, annual ryegrass/Lolium multiflorum, oat grass/Avena sativa. You can plant the seeds in large containers and keep the area free from chemicals. That way you know the dog will be okay.