Well, as a brit I can say that other than Majesty for King/Queen, Highness for Princes and Grace for Dukes I haven't got a clue about the correct forms of address and protocol. Then again, being a soldier I have the great cop-out of everyone in nobility terms being addressed as Sir or Ma'am by the non-coms and junior officers, as a general rule. Most TV newscasts of visits I have seen have most of the normal folks calling them Sir or Ma'am, and only the people who, as Gallowglass said, have reason to know the protocols were using them.
It makes sense for Traveller to have that sort of traditon maybe, that military folks call nobility Sir or Ma'am exclusively until they reach Staff Officer ranks (i.e. Brigadier and above in british army terms). After all, Sir or Ma'am is used to address officers and *ahem* betters, so its still a mark of repsect, especially 'cause most (Traveller) nobility hold military rank anyhow.
Take care,
Shane