Essentially, ETTR is the digital complement to the Exposure component of Ansel Adams' Zone System -- capture as much information as possible in the exposure, with the intention of working with that information in later stages to create an image. It is accomplished differently in digital than in film, i.e., expose to the right instead of expose for the shadows, but the objective is the same.
Understandably, automatic metered film cameras never incorporated this concept, because the expectation was that most users would simply opt for straight processing and printing. Likewise, for digital cameras, the designers' assumption for automatic modes is that the camera should capture an image that is as close as possible to a printable image, which gives up the benefits of having an initial image recorded that includes as much information as possible for later redistribution. Camera makers are unlikely to design their cameras to overexpose without clipping by default, because such images will look terrible if they are simply printed or pulled into iPhoto or Picasa. Unless there is a exposure-boost tag that automatically causes the standard photo programs to crank down the exposure by the same amount, pictures will look without manipulation.
Perhaps a more useful approach would be to use ETTR metering techniques when recording RAW images, on the assumption that anyone using RAW will be processing images individually and will be positioned to take advantage of the improved data capture.
