As I understand it, the curve is virtually mandatory and significant pressure is exerted on professors of large classes who consider not following it. Technically professors have discretion to grade as they see fit (I've seen that e-mail too), but as a practical matter the curve is mandatory for all the major courses. I don't know of any large mainstream classes that don't follow the curve. Classes that are unusual, not really law-based or near the size cut-off may vary a bit. I've had one class that was large enough that it was supposed to fall under the recommended/mandatory curve where the professor openly disregarded the curve (and it wasn't a strictly legal course). Otherwise, the curve appears to be understood by the faculty to be mandatory based on my sampling of courses. My sampling of "pass fail" courses is fairly limited though as we had normal grades my first two years. I suspect professors who stuck with the curve for real grades will be even more inclined to stick with the curve with the pass/fail system.