At the end of season two (episode Doomsday), when the Doctor says goodbye to Rose at Badwolf Bay, she tells him that she loves him. He replies, since it is his "last chance," "Rose Tyler..." and is cut off. The natural assumption is that he was going to finish with "... I love you" but I have an alternative theory. At the end of season four (episode Journey's End), when the Doctor is leaving the duplicate Doctor with Rose in the parallel universe, she reminds him of his unfinished declaration and asks him to finish what he was going to say. The Doctor states that it doesn't need saying. When she asks the Duplicate he leans into her and whispers into her ear. Again, the natural assumption is that he was going to say "I love you" but why the whispering then? She immediately kisses him passionately and he returns the kiss so it wouldn't really be a secret. If you think back to the episode Forest of the Dead, when Professor River Song whispers into his ear it is later revealed that she was whispering his name. Is it possible that the Duplicate Doctor was whispering because he was telling Rose his true name? I've got nothing else to back up this theory but it seems like the kind of ambiguous thing the writers of Doctor Who would put out there.