It's an initialism that stands for Don't Forget to be Awesome. It was made popular by the community that follows the vlogbrothers on YouTube. http://www.youtube.com/vlogbrothers
While we would entertain a DFTBA design, the artist donated all proceeds to charity so DFTBA was behind it 100%... otherwise we can't legitimately run a shirt that has "DFTBA" on it
You are right OEB, we have many Who shirts, StarWars, Star Trek etc etc etc, why not another DFTBA? :)