Back when 4e was first announced, the first question I asked Keith Baker is "How will Tiefs be handled in Eberron?"
He suggested that Tiefs are what happens when someone is born when a plane is cotenous, or to some births inside a manifest zone. Not only is the tief's birth connected to that plane, personality is also; a Tief in line with Firnia is likely to be quick tempered and an arsonist; a Tief connected to Shavarath is warlike and eager for violence.
Honestly? I'd say that the Dragonborn are a creation by the Rajahs. During the War agains tthe Dragons, the Rajah created the DBs as 1) A mocking insult to actual dragons, and 2) as foot soldiers. When the Rajah were defeated, the Dragons sealed the DB away somewhere in Khorvaire, in a state of eternal slumber. Within the last decade though, something has caused them to slowly resurface.
Otherwise, I'd put them in Q'Barra, and say they a new race, created by lizardfolk births in proximity to the dragon guarding that ancient city. There's no inter-species hanky panky, so much as it is proximity to the supernatural nature of dragons effecting the souls and bodies of lizardfolk eggs.