I've come across various business cards where people put some message on the back about their product or their vision
Has anybody tried this with free software principles?
One issue is how to condense the text to fit within the available space - the full DFSG text may not fit but maybe the 4 principles here (just the text in bigger letters): http://fsfe.org/about/basics/freesoftware.en.html
An alternative may be to develop a question that people can print and underneath put a link to FSFE, e.g.
If the software and apps you rely on are not fully open and transparent with their code, are they serving your best interests or somebody else's? http://fsfe.org/your-best-interests