A DOF scale correlates each aperture with an estimate of the DOF that will be available on either side of the focal point at that aperture. The problem is that a zoom lens introduces a new variable: focal length. The DOF scale would have to be a sliding scale that changed as you changed the focal length. For example, the DOF for f/8 will be very different when the lens is at 70 than when it is at 200. See here
Notice the word estimate above. Even if you have a prime lens with a very authoritative-looking, very precisely marked DOF scale, the meaning of the scale is entirely dependent on the choice the manufacturer made for a certain variable called the circle of confusion. See here
for more on that. In general, DOF scales on lenses are way to forgiving for today's standards of print sharpness. Many photographers routinely use the DOF reading from one or two stops wider aperture to compensate. IOW, look at the photo of the DOF scale on the Canon 50 lens in Norman's tutorial. If you wanted the DOF for f/11 you would read the distances opposite the f/8 or even the f/4 marks instead.
does the depth-of-field preview button compensate?
Yes, that's the intent. When you look through your finder normally you are seeing what the lens sees when it is at its maximum aperture. For the lens you mention, the 70-200, the max aperture is f/2.8. You are seeing the DOF that f/2.8 gives. When you take the picture however the aperture stops down to the f/stop you've selected. The DOF preview button simply causes the aperture to stop down while you hold down the button. While you do that you are seeing the DOF that results from the selected f/stop, which is the same thing that will be recorded on film.
The downside is that when you press the DOF button you are narrowing the lens aperture and thus reducing the amount of light that makes its way to the finder. You'll have to wait a few seconds for your eye's iris to dilate before you can see clearly in the reduced light. One aperture is opening to compensate for another aperture closing!