Q:

Addressing a former office-holder by that office's title

When is it appropriate to use an "expired" honorific to address or refer to a person? 
In the U.S., former state governors are occasionally referred to as "Governor So-and-so", although they have not held that office for several years. I see this happening most often when the former governor is actively campaigning for a new office (usually U.S. President), and my impression is that it's done only by someone with an interest in the campaign (for or against) rather than a (relatively) neutral party like a newspaper.
Certainly this could be used as a jab: in a political campaign for high office, addressing one's opponent by a former title — lower than one's own — could imply that the opponent is not qualified for the new job. It could also be used the other way around, to remind listeners that a candidate one supports used to hold an important title.
I'm curious whether there are formal rules for this in the U.S., or at least style or journalist guidelines, for various offices. Are ambassadors, for example, entitled to some sort of emeritus honorific?

A:

I would agree that this is more of a question of protocol than language, but I suppose it is an example of how usage changes, and formal protocol and "polite" language deviate.
The traditional rule is that for offices held by a single person at a time (e.g. president, governor, mayor), a title should only be applied to the current office holder. If the office or rank is held concurrently by multiple people (e.g. judge, professor, and military ranks — although usually only for senior officers), the designation is retained for life.
Once out of office, the individual reverts to whichever title or honorific applied before he or she held office, although as a courtesy, "once an Honorable, always an Honorable." Thus, Governor Howard Dean of Vermont, "by the book," became upon retirement Dr. Howard Dean, and would be addressed as Dr. Dean, but might be introduced as The Honorable Howard Dean, former governor of Vermont. President Jimmy Carter became Mr. Carter (or even Lt. Carter, although ranks of retired military are usually applied only for senior officers). As far as I can tell from perusing other official governmental protocol guides, this remains the formal practice in the U.S.
In common practice, very high officials such as the U.S. president have been granted a "courtesy title" for many years now, and the "courtesy title" is trickling down. Living just outside Washington, I tend to blame commentators on 24-hour television news networks trying to secure interviews from former public officials by feeding their egos, and wish I could think of a more elegant phrase than "obsequiousness escalator" to label it.
Not a few columnists and bloggers have expressed indignation over the extension of courtesy titles, with more than a few indisposed to extend the courtesy to "Speaker" Newt Gingrich.

I'll add a quote from a U.S. Department of State diplomatic protocol guide

Over the years, and recently as well, there has been discussion about the use of the honorific title of Ambassador by former ambassadors, both those who remain active in the Foreign Service and those who are  retired. For years, Department regulations have forbidden this usage unless actually in the job  of ambassador or for those few who retired with the personal rank of career Ambassador.
For current employees, long-standing custom and practice, however, has established a clear tradition in the Department and in the Foreign Service that persons who have served a ambassador after Senate confirmation may continue to use the title after such service in appropriate communications with others, may be referred to in communications and  conversations by the title of Ambassador, and may be introduced to public audiences by the  title.
The Department has also clarified the use of the title for persons who have retired from the Foreign Service or left government service who served as ambassador after Senate  confirmation. An amendment to the various regulations permits the use of the title,  “Ambassador, Retired,” for all such persons.

A:

In the United States, the formal rules are that only senators, governors, presidents, and officers in the armed forces  retain their courtesy titles after leaving office.
Nobody else.
I heard the rule explained by legendary newsman, the late Daniel Schorr, on the air some years ago.  I don't know whether it's in his books, of which I have only one. He used the Powell example specifically as to whether his title was General or Secretary upon leaving office. I'll try to find other sources later.  I know that members of the US House of Representatives, like cabinet secretaries and ambassadors, are not supposed to be given their titles once they leave office, and that if they want one, they need to become a senator, governor, or president.
That means, for example, that Colin Powell's correct title is now General, not Secretary. That's because he retains his rank of general even afterwards, but secretary is not a rank and he is no longer deserving of the title.
Protocol does not require that you address former secretaries, representatives, ambassadors, or other functionaries by their former titles.  Indeed, doing so  was formerly considered a breach of protocol.  An ex-ambassador is not Ambassador Anybody. 
However, these days, no one pays attention to such niceties, which means that everybody pretty much gets called whatever.  Just ask them how they wish to be addressed; that guarantees that you won't address them in a way contrary to their own preferences. Whether it bothers anybody else, well, that will always be true. 

