I've been wondering about this for a while.
My understanding of the etiquette of titles is that a former U.S. Senator may be addressed as "Senator," but-- for reasons that are not clear to me-- a former U.S. Representative is not addressed as "Congressman" or "Congresswoman" or "Representative" So-and-So. (In fact, a sitting Representative is addressed as "Ms.," "Mrs.," or "Mr." as appropriate.)
On TV the past few months, I have seen political speakers on the podium, and pundits in the studio, refer to former Representative, and former Speaker of the House, Newt Gingrich as "Speaker Gingrich." Sometimes I have even heard him addressed as "Speaker Gingrich." Seems to me this is incorrect.
Only one person at a time is Speaker of the House. In the third person, a former Speaker should not be referred to as "Speaker Gingrich" although "former Speaker Gingrich" is a suitable description. In the second person, the former Speaker should be addressed as "Mr. Gingrich." (If he had been a Senator, it would have been correct to address him as "Senator Gingrich.")
Mr. Gingrich has to just sit there and smile when he is addressed incorrectly, since it would be unseemly to correct someone who is attempting to honor him. In his shoes, I would do the same. But it bugs ME to hear this.
I don't know how far to trust this guy, but he has firm opinions on how to address former officials.
My understanding of these matters may be clouded; if so, I invite clarification.