Paul,
Search the boards a little and you'll find the answer to your question in more detail.
Shortly: The actual "passing score" for the PMP exam is a closely guarded secret at PMI and no one knows what it is. Also, with their testing method, your "passing score" will be slightly different than the person who is taking the test next to you. Tests are computer generated and each question has a difficulty weighting associated with it, so the true passing score will vary somewhat person-to-person.
A general thumbrule for the PrepCast Exam Simulator is that if you're consistently punching out 80%+ on full exams, you're probably well-prepared for the real PMI exam. I can't speak to other exam simulators and their thumb rules.
Good luck
