rocknbil - 7:11 pm on Apr 28, 2008 (gmt 0)

From the help files (OLD version!)

You can add copyright information to Photoshop images and notify users that an image is copyright-protected via a digital watermark that uses Digimarc PictureMarc technology. The watermark—a digital code added as noise to the image—is generally imperceptible to the human eye. The watermark is durable in both digital and printed forms, surviving typical image edits and file format conversions—and is still detectable when the image is printed and then scanned back into a computer. ......For more detailed information on embedding Digimarc watermarks, refer to the Digimarc Watermarking Guide, downloadable for free from the Digimarc Web site at www.digimarc.com.

This is probably part of the later version distributions as a plug-in.