I blogged a whole series on Christine Whelan's 2007 book, Why Smart Men Marry Smart Women, starting here. She has a new study advancing this point, with a nifty chart of men's changing preferences in women.
I believe this change is especially prominent in the educated classes - proving the point that smart men marry smart women in a double sense.
(From "Measuring Mate Preferences: A Replication and
Extension" by Christine B. Whelan, University of Pittsburgh, and
Christie F. Boxer and Mary Noonan, University of Iowa).