Well, not totally ;). They just think I don't watch them. My yard is 1.5 acres and I let my 9,5 ( almost 6) and my 3 ( almost 4) year olds go outside without an adult. My 3 year old is required to stay in the back yard where I can see her at all times but the other 2 are allowed to all over the yard but not near the driveway. They listen or the immediately come inside. In general they are very good at doing exactly what they are told. They can stay out as long as they want so long as they come in for meals/snacks and whatnot.
I love watching them have fun without parents hovering. They are very good at entertaining themselves and my 5 year old is always bringing me bugs and toads. Do you allow your kids to have 'unsupervised' play?