Just booked a flight on United.com for an Aug trip. The SEA-NRT leg I picked the ANA flight of course, since it was the same price as UA operated one and the schedule virtually the same. Although I am disappointed this won't be on the new 787...
Since the 787 is planning on being used I don't think *A is expecting too much traffic between SEA and NRT, but since the schedules are basically the same, I don't see why just either UA or ANA runs this route on a A380 or 747 and the other codeshare.
As a side note, I think UA will need more flights to SEA to feed into the two flights running to NRT now.