I'm not sure how you define CS0, but my understanding is that there are now three introductory CS courses at Harvard: CS1, CS50, and CS51. Note that I graduated in 2007, so my understanding may be dated.
CS1 is intended for non-concentrators or people with very little prior CS experience.
CS50 is the standard intro course that most CS concentrators (or people considering CS as a concentration) take. Although it's taken by non-concentrators, most of the non-concentrators taking the course do so because they're in some field that has a lot of exposure to programming (e.g. Applied Math).
CS51 is the "advanced" intro course and is usually not the first CS course a CS concentrator takes, if for no other reason that it's not offered until the spring semester.
CS1 is intended for non-concentrators or people with very little prior CS experience.
CS50 is the standard intro course that most CS concentrators (or people considering CS as a concentration) take. Although it's taken by non-concentrators, most of the non-concentrators taking the course do so because they're in some field that has a lot of exposure to programming (e.g. Applied Math).
CS51 is the "advanced" intro course and is usually not the first CS course a CS concentrator takes, if for no other reason that it's not offered until the spring semester.