Pat, as a die-hard Old School Top Secret RPG fan, could I ask you to at least consider including Assassin, Burglar and Investigator type agent classes (or equivalent) please ?

Class mapping has been the focus of many conversations of late, and I'm happy to report that classes are better conceived for this game than anything we've ever released. I imagine we'll go into greater detail in some kind of designer diary down the road, but for now allow me to say simply that every class for Spycraft Third will not just speak to an archetype you may want to play, but will speak directly and intimately to a major mode of play.
That statement will make more sense when you see how Spycraft Third is structured. There's a methodology at work here that helps everyone, players and GMs alike, and goes beyond merely streamlining the game to make it easier to teach, write and design for, and run as well.
And yes, you'll be able to play assassins, burglars, and investigators. In fact, those are all tied directly to said modes of play. (In fact, they're examples of three of them.)