I was given a link to this article in Scientific American, about preservation issues and resolutions for Daguerreotypes. To read the entire article you need to subscribe to their website, or purchase the hard copy at a newsstand. The gist of it is that there seems to be a problem with displaying Daguerreotypes where they are under significant continuous exposure to light, and it has to do with the way they were made. With exposure to light and oxygen, a reaction occurs on the surface of the image that begins to form a whitish fog on the plate. The long-term solution appears to involve sealing the images in an argon-filled frame. The short-term solution, or for those who don't have the budget of a major museum's conservation department, is to keep them in dark storage when not actively engaged in viewing the image(s) and to severely restrict exposure to light when on view.