I may've said this before or it may've been buried in this thread, but I'm fairly certain that Apple is going wild hog for the daughterboard system so they have higher margins; in this case, for marketing technique, I would say that they're waiting for lower-clocked W36xxs to release for the MacPro to offer a steady lineup instead of a quad/hexa release for optimal PR.
Of course, just a gut feeling, but in my theory Apple is holding themselves back as Intel hasn't released a whole new line; Intel introduced a whole new line for the Nehalem release, but not for Gulftown (single-socket-parts). In the dual-socket realm, having differing clockspeeds was essential (as many server farms I imagine have electrical and heating profiles that they have to fit and the need for cores).