Steven F , Jan 26, 2010; 08:19 a.m.
What Ansel Adams wrote decades ago may or may not 100% accurate for cameras today. Camera design has changed significantly since he died. Additionally not all exposure meters are made the same. Many older cameras out there don't care about color. Some of the newer ones do take color into account. When Ansel Adams started out in photography light meters were not available. Digital sees light differently than film and Ansel Adams never worked with digital. What may be true for Nikon may not be true for Canon or Pentax.
Additionally what shows up as gray on the camera can look significantly different on the monitor. Before making comparisons between your monitor and printers they should be calibrated so that they all render colors the same.
For most people "where is middle gray really?" is not as important as using the camera and getting experience with how your particular camera on how it handles various lighting, color, and white ballance situations. Most photographer are more concerned with the final look of the image rather than if the exposure exactly matched the value from a gray card. In fact many people under or overexpose a pictures deliberaely to get the look they want.