[math-fun] Earth = mathematical ellipsoid ??

One of the standard GPS models of the Earth is that of an "ellipsoid", where a cross-section through the N-S pole is an ellipse. http://en.wikipedia.org/wiki/Reference_ellipsoid But I seem to recall that a fast-rotating body of liquid produces a funny discus-like form. Given a molten liquid of "constant density" -- e.g., mercury -- held together by Newtonian gravity, what mathematical shape does it assume as it rotates faster & faster? How fast can it rotate before the ellipsoidal shape no longer applies?