Yes - well, sort of, albeit (as far as I know) unwittingly. I attended a conference where one of the relators was prof. Jessica Fridrich reporting on the identification of digital image sources. You can probably start your search on her page, here.
To be clear, this is nothing like intentional yellow-dotting performed by some printers, and cannot be used to trace a picture to a camera unless you also possess that camera and can take other pictures to compare.
What happens is that every CCD chip has subtle variations that can be recovered through frequency analysis from the original picture data, and these variations are manufacturer and chip specific. This allows, with a certain error margin, to:
given two pictures, determine whether certain areas of one picture are original or whether they come from a different source (the patterning in those areas is not consistent with the CCD)
given two pictures, or a picture and a camera, determine whether the picture came from that camera or not.
It would be extremely easy to make this system more reliable, by e.g. laser etching the CCD chips with a unique pattern. This would not be visible to the naked eye when inspecting the pictures, but the information could be recovered.