Yeah, I'm pretty sure they were talking about Frank being a candiate for Jacob to come and use his body/form, like "the man in black" did to Locke.
Remember in season three when (the real) Locke caught that glimpse of Jacob in the chair? That certainly didn't look like the Jacob we now know, but it *did* kind of look like Frank, right?
Okay, so maybe that takes some imagination, but that's just my guess. Maybe with some crazy time travel stuff Frank ended up being the Jacob he saw?
Then again, another theory floating around has been that it was really Jacob's enemy/Esau/The Dark Man in the cabin, and not Jacob at all...
But who knows???
They could have also been talking about him being a candiate for becoming one of them/an Other/ on the List.
