[Pitchwife]

Interesting question indeed! I guess my take on the matter would be that there are elf-friends and elf-friends, so to speak.

The heroes of the First Age Elrond named at the Council were elf-friends in the sense that they were allied with the Eldar and fought side by side with them in the war against Morgoth (if with somewhat unhappy results in Túrin's case), so I think the title was honorary in their case, recognizing their valour and their service to the Elven kings who were their liege-lords. The Orcs in The Hobbit apparently used the word in the same sense (just from an inverted perspective): people who are in league with the Elves (as evidenced by their use of Elvish weapons).

The special quality Goldberry perceived in Frodo seems to be a different matter. I don't see it so much as being caused by the naming and blessing, or something born into a person like Findegil suggested. Rather I imagine that when somebody is in frequent contact with Elves (and we know that Bilbo and Frodo would try to meet some whenever they had a chance), something of the Elvishness, the spiritual quality of the Elves themselves, would sort of rub off onto them; and when Gildor named Frodo elf-friend, it was more a recognition and quasi 'official' confirmation of a quality Frodo had already aquired by his previous contact with Elves (and presumably his love for Elven lore and matters).

As far as elf-friends of this second sort are concerned, I whole-heartedly agree with what Legate said about 'good luck' and the ability of others to percieve them; and great thoughts about Saruman in this respect!

Mixed cases are possible, of course, in the sense that elf-friends in the first sense could (and would be likely to) also become elf-friends in the second; among the examples Elrond quoted, this would certainly be true for Beren, probably Hador and Húrin as well, maybe even Túrin.

Bilbo is another mixed case, I think. When Thranduil first bestowed the title on him in The Hobbit, it was probably just honorary, given in thanks for his efforts to make piece between Thorin and the Elves; but by the time Bilbo went to live in Rivendell, he had also become an elf-friend in the second sense.