Thanks to the investigations kick-started by World of Warcraft player Sendatsu, it's been discovered that Activision Blizzard has been embedding hidden watermarks into screenshots taken with the game client over the past couple of years. The exact time the practice began has not been settled-upon, but it's believed to have begun somewhere during 2008 with patch 3+, and 2010 with patch 4+.
It hasn't been mentioned how Sendatsu stumbled-upon this discovery, but simple instructions are given to allow you to verify the practice. You must find an area where a texture is as flat in color as possible - white ideally - and it's better if you're able to zoom in so that the solid color fills your screen. One recommended area to do this in is below Dalaran in Crystalsong Forest. After a screenshot is taken, you can sharpen the image to such a large degree, that the secret watermark will become visible. An example:
It might look like gibberish, or an artifact as the result of the game's JPG compression, but with the help of others who joined in on the investigations, example applications have been released in both Java and C# that scans the watermarks and decodes in the information within.
That's right - these are not simple watermarks. Information that can be decoded from them include the date and time, realm, account ID (eg: 107642169#1) and server IP (which would be useful to Blizzard in searching for private servers). This information does not contain your account password, your IP address or any other sensitive information. In the highlighted image below, blue represents the date and time (sans seconds), while the red includes everything else.
As of the time of writing, Activision Blizzard hasn't spoken up about its practice, but with news of it spreading fast, the company isn't likely to remain quiet for too much longer. While no truly sensitive information can be decoded from these watermarks, those who have been investigating the matter don't like the possibilities of what could happen.
It's important to note is that the watermarks do not apply to screenshots that are captured in the highest quality (/console SET screenshotQuality "10"), so if something about an embedded watermark rubs you the wrong way, it's recommended that you double-check the setting the next time you log on.