According to wikipedia, 博士 (hakase) is used as honorific when addressing some academics:
Sensei (先生、せんせい) [...] is used to refer to or address teachers, doctors, politicians, lawyers. [...] The term is not generally used when addressing a person with very high academic expertise the one used instead is hakase (博士, lit. "Doctor" or "PhD").
However, this really does not match my experience from when I was in Japan. I never heard anybody refer to a person holding a PhD as 博士, especially not for professors where 先生 was always used. That said, maybe this was owing to the more international setting I was in.
I'm wondering in which settings 博士 is actually used as a honorific, if at all, or if the information on Wikipedia is simply outdated or incorrect.