In 2001 Wired had an article about a company making a pen that could read microdots on paper while you are writing. It would then be able to uniquely identify the paper that you wrote on and correlate what you wrote with, for example, the form you wrote it on.
I'd wondered a few times what happened to that company (Anoto), thinking they are probably long gone. Turns out they are around, and the pen they are using in this article is actually one of their models. Not sure if it is still using the microdot idea though.