I just stumbled upon this page, which might be of interest to others here.
Imagine that every time you printed a document, it automatically included a secret code that could be used to identify the printer - and potentially, the person who used it. Sounds like something from an episode of "Alias," right? Unfortunately, the scenario isn't fictional. In a purported effort to identify counterfeiters, the US government has succeeded in persuading some color laser printer manufacturers to encode each page with identifying information. That means that without your knowledge or consent, an act you assume is private could become public. A communication tool you're using in everyday life could become a tool for government surveillance. And what's worse, there are no laws to prevent abuse.
(from above linked page)