I imagine the used market will probably see a decent drop in price. But, considering how similar the D3 and the D3S are, performance-wise, that might actually be questionable. There will probably be some discounts to get the D3 off the shelf, but it's still such a coveted camera that it will still remain pretty high.
I do know that the price of a used 1Dmk3 dropped by about almost a grand with the announcement of the mk4, but the D3 hasn't dropped by much yet. On another note, the D2X dropped by almost a grand as well.