I resurrect this thread (on its anniversary) not only for its relevancy to the twist taken in Dr. Bill's Fabulous Word List thread in I&A but because I wanted to comment on Bobyb's observation:

The French "vous" used for a polite form is the plural, but it didn't come to be the polite singular form the same way the English plural came to be used for singular. It comes from the old practice of Highly-placed Persons referring to themselves in the plural ("We are not amused") and their inferiors addressing them in the plural.

This is exactly the way Shakespeare used the words thou/thee and ye/you. Read his plays and notice that when people are speaking up (e.g., peasant to lord or lord to king) they use ye/you but when speaking down they use thou/thee.