I personally have never tested the machines, but I don't believe they are correct on their speed. I starting running on the treadmill for about 2 weeks. After those 2 weeks I realized I was almost an olympic runner as I could run a mile under 5:30 mins. Now granted I've always been in good shape, but I've never broken a 6 min mile on a real track.
I talked to a friend of mine that trained on treadmills in high school to get him in better shape for football, and he said there is no way the mph on the treadmill is the same. His logic was that since the treadmill is moving the ground for you, your not getting the full resistance that the street would provide. To me that makes sense as I seem a lot lighter on my feet on the treadmill than on the street.