I was asked by a friend whether digital watermarking is really effective as court evidence, so I did a quick research. There is one case recently decided (Feb. 2005) that seems to take it for granted that digital watermarking is good enough for claiming the identity of digital data.
See, Palladium Music, Inc. v. EatSleepMusic, Inc., 398 F.3d 1193 (10th Cir., 2005) at 1195. ("Employing embedded digital watermarks, Palladium was able to determine that the files ESM was distributing online were originally sold to TPC for its CD+G line of products. Following this discovery, Palladium brought this action against defendants ESM and TPC alleging copyright infringement for marketing Palladium's sound recordings online without a license.")