My take for what could happen until next year :
1) The D4 was announced but won't be available for the majority for a couple of months.
2) New D3s bodies still sell for 5000$ + , there isn't much stock left
3) Used D3s have kept their value because the D3s sensor isn't available in a smaller body (think D3 -> D700)
4) The D4 is a major upgrade for video, but my guess is it won't be a huge improvement in image quality (12->16mp like D90->D7000, slightly better high iso quality but not as much improvement as from D2 generation -> D3)
Today used D3s run between 3,500$ (very hight shutter count) and 4,7000$ (like new, low shutter count).
My estimation: next year, the D4 will be around 5,800$ new and the D3s around 4,000$ used but in excellent condition (maybe as low as 3,000$ whit a high shutter count).