It seems the Blizzard Entertainment is able to track World of Warcraft players to a limited extent using information embedded in any screenshot captured by the WoW client. The discovery was made when one player noticed some strange artifacts in an in-game image he had grabbed using the built-in capture feature, as posted on Slashdot Games (via GamePolitics).
After sharpening the image, the WoW player noticed "a repeating pattern secretly embedded inside." He then went and shared his discovery with Owned Core, at which point a number of community members set to the task of figuring out what this meant. They soon learned that the watermark reveals the user ID and IP address of the person who captured the screenshot as well as the time it was taken.
The Slashdot poster points out that the real issue here is that Blizzard never notified its users in its terms of service that they could be tracked in this manner. The World of Warcraft developer has yet to comment on the matter.