Sluggo said:

I can't believe I'm joining this argument, but when 3PO says to R2 that they are trying to save Han from the bounty hunter, keep in mind that he was addressing R2. If ol' goldenrod was talking to a human, he may have used 'Captian Solo' instead.

To the extent I ever thought of it (very small) it seems to be a little pay off to the bonding going on in the movie. It starts with Han leaving the Rebellion, but ends with "You have to take care of her now" and 3PO is part of that family that was formed. So in the moment he calls him "Han."