Without digging out a book, I agree with Eric and Matt on this.

One thing I don't see mentioned here is emphasis on the fact that "penalty" turns are meant to be just that. They must be in addition to (i.e. not inclusive of) any turns that you would make to sail around the course.

Looking at the diagram above, pretend there's a weather mark in the middle, and the boat is sailing the other way (approaching on port). If that boat fouled another (typically a starboard boat), she cannot simply tack, round the mark, gybe and call that a penalty, primarily because the tack was a normal turn as part of sailing the course. If other boats were around, the gybe would also fail to count, as she would not be clear of other boats.

Mike