Okay, so the preview for next show, "The Return", shows a quick clip of what looks like Pinocchio, in fairy-tale land, getting zapped by some kind of power from the tree that he and Gepetto are preparing for Emma.
What if the tree, during its conversion to a transport system, somehow absorbs Pinocchio's nose-grows-if-he-lies abilities and passes some of it on to Emma when she goes through to the real world? Is that why she has a lie-detecting superpower?
More wild theories - I've seen other people theorizing on discussion boards that August is Pinocchio. If we consider that something went wrong while Pinocchio and Gepetto were preparing the tree for Emma, this could well be possible - maybe Pinocchio was transported by accident to the real world? Maybe that's how the tree absorbed his nose-grows-if-he-lies powers to later pass onto Emma when she went through? Maybe this is how August avoided the curse and retained his memories? (Although that still wouldn't explain how he knows about Baelfire and Rumpelstiltskin's knife. But it would explain how he knows about the Saviour.) Maybe going through the tree prematurely is what made August sick?
This theory would certainly fit with August's apparent age - Pinocchio looked about seven when the curse was imminent, plus twenty-eight years in the real world would make him thirty-five...yeah, August could pass for thirty-five.
And here's another what-if...what if August/Pinocchio was the kid who found Emma on the side of the road when she came through? This would give them that "history together" thing that August kind of let slip in a previous episode, and which Emma looked confused about.