Me, I've always assumed that if any of the companions weren't going to be announced, then those companions wouldn't be included in the nine that we know we're going to have. Seems to me that if you want there to be a secret companion, they need to actually be secret.
That being said, seeing as how the right hand(the devs) doesn't know what the left hand(marketing) is doing, who knows.
I've seen DG say once that if it was up to him, none of the companions or romances would be revealed before release because he wants us to enjoy the surprise of discovering them ourselves. Personally, I found that to be a bit presumptuous, assuming that we'd even buy the game without having a reason to.