Using Bieber is terrible idea. His audience is worst possible fit for bitcoin and it is too big. It may boost bitcoin user base, but too fast and will bring concentrated amount of targets for hackers and scammers.
More damage than good.
If you want celebrity endorsement you need to choose very carefully and start smaller.
I would propose Stephen Fry. He is a tech-savvy guy and has good enough following not to overwhelm. He is more of intellectual type of celebrity... And he supports open source.