I think '21 grams ' may have done it.
Perhaps the fact that it's hard to tell is a sign of either it's limited use or it's lack of success. Either way, the concept is fairly simple and much like a technique used in the silent film era. Back then scenes would sometimes be tinted to infer a mood. A light hearted scene might be yellow , whilst a scary scene would presumably a darker colour. The difficulty is that colours convey so many different feelings it is difficult to read, thus the whole process is useless unless you know the language.
However, if this principle is applied to grain everything changes for two reasons:
1) We know the language. Reporters have always used cameras that decrease in quality as danger rises.
2) Grain is simpler than colour. Either it's not there, or it is in varying amounts up until it is the complete image.
So not only could grain be used to enhance a feeling of realism, but it could also be used to express a characters state of feeling. When the character is at a high point the image would be clear whilst when they are at a low the image would be grain heavy.
This is, of course, a very formalistic technique which in some regards reduces the role of an actor by replacing them with a specific device. But this isn't necessarily a bad thing. Editing is, afterall, the prime technique used to deliver information in cinema.
