I received a used Canon 1Ds Mark 3, advertised as having a shutter count of 3900 actuations (which is extremely low, if true this would have been a camera that had seen little use and be virtually new).
The seller informed me that this was determined using Exiftool, which can be run on any JPEG file. I obtained this tool and ran it on several JPEG files from this camera and obtained the same number: 3900. And it never changed, despite my taking numerous images. Clearly this field, which may be accurate for other camera models is not providing the shutter count on the 1Ds Mark 3.
Some people say that this data is "Canon Proprietary", and maybe it is. In any event, it is not something the end user can obtain on these cameras. Also it is possible to transplant a shutter count from one camera to another by saving settings from one onto a flash card and then restoring them onto another camera. This opens up the possibility for all kinds of fraud. An unscrupulous person could "turn back the odometer" once they figure out what field on the CF card to modify.
Tom's Digital Photography Info / tom@mmto.org