While reading a review on Canon's new digital camera, the EOS 20D, I noticed that the camera supports a verification kit for proving the authenticity of digital photos.

Support for DVK-E2 data verification kit
The EOS 20D becomes an option for law enforcement and any other industry which requires verification of image authenticity. The camera can now embed a unique encrypted signature into each image which enables them to be later verified using the DVK-E2 kit for authenticity.

I googled on the "DVK-E2" and found a press release back in Jan 2004 which describes some applications for the kit.
With the capacity to detect the slightest (single bit) discrepancy, the DVK-E2 kit is expected to be popular amongst law enforcement, insurance, news and other agencies that have a need to verify the legitimacy of digital images.

It would be interesting to find out if any digital photographs were submitted as evidence in a trial.