MysticCat, I've found mostly bloggers' take on the bow, but I do remember hearing about it on NBC. This
link at least quotes a White House spokesman, Robert Gibbs.
One excerpt that I feel compelled to cite: "But the traditional Miss Manners book of etiquette advises: 'One does not bow or curtsy to a foreign monarch because the gesture symbolizes recognition of her power over her subjects.'"
I have no idea if this source has an agenda or not.
Loved the Jackie Kennedy story!