In most grad schools, any form of C (or worse) is considered a failing grade. I think the theory is that by the time you get to grad school, average (C) isn't good enough. But, honestly, they really could just scale the grad school grades to mean anything if they wanted to.
Most grad programs require a minimum GPA of 3.0 to stay matriculated or to qualify for financial support, so technically B- is consider failing. However, professional schools typically do not take into this tidbit of information into consideration.