Old photographs.
Ars Technica had a newspost a ways back about the traditional film manufacturers falling on hard times. While somewhat true, film isn't in any danger of disappearing--there are far too many artists, prepress operations, X-ray machines, and consumers in general who purchase film for the medium to truly disappear. Further, the arguments on the forums moved between those arguing that digital is a much better medium than film and vice versa. What I would like to comment upon is a point that lends more credence to the idea of film as a signifying medium over digital.
One of the primary draws of digital photography is the idea that the digital source file will last forever. A digital copy is after all a stream of numbers; the numbers do not change, and barring a catastrophic data loss, those numbers will never age, corrode, bend, or yellow.
It is this factor that is digital's greatest detriment.
In fact, it is an error to suggest digital source files never change. The streams of numbers perhaps are constant, but the processing method itself is open to revision. Apple Computer's own Aperture software processes RAW files through calls to an API (application programming interface) called CoreImage, and CoreImage is a primary component of Apple's operating system. Thus, as the operating system changes, so does CoreImage, and the way your RAW files are processed changes, as well. It is the colorspace that is shifting around you.Film does not suffer from this particular movement. As an "analog" medium, the way our brains process light does not change in as significant a fashion as the recreation of the colorspace represented by changing the CoreImage API.
That is not to suggest that film's colorspace never changes; far from it, in fact. Whereas the digital file will change colorspace, the medium the photograph is printed upon will physically deteriorate. Anyone who's seen old Daguerreotypes, or even forty-year-old silver halide prints, can recognize the damage that time wreaks on a photograph. The aforementioned yellowing, bending, cracking, fading, streaking, folding--all these will happen naturally, without user action. Combine this with the recognition that it is fundamentally easier to alter a negative than a digital source file (say, by leaving them within reach of children), and film begins to look more fragile than its modern cousin.
However, the ways in which film ages adds semiotic content to the photograph. That is to say, when a photograph ages, it does so in ways that add value and unspoken meaning to the photograph itself. Old photographs are valuable only partially because of their semantic content--the actual subject of the photograph--and the remainder of that value and meaning is presented by the way in which the photograph has aged. Far from being a detrimental process, the aging of a film print is something that creates another layer of signification on top of that the photographer originally intended.
One can, of course, argue that printed digital files will experience the same process. And of course they will. But we're no longer talking about digital files, when one is pointing to an analog print. That's back into the realm of film, and rather than being a rebuttal, is a point in film's favor. (Whether this argument is, in fact, a tenuous straw man is left to the reader to decide.)
Rather, when digital files "age," when their rendering APIs are rewritten and you find your colorspace altered, has it added semiotic content to the photograph you originally took? I submit that it does not. Instead of adding a layer of signification onto the original work of art, instead the aging digital file has interfered with the production of that work. You have added a layer of noise when the signal itself is too precious to compromise.
This is possibly a long-winded way of saying that I will give up my 35mm camera, and my Soviet TLR, only in my last will and testament.