I don't think Rosberg is going anywhere. Williams are really improving - they finished ahead of the Brawns on merit in Germany and ahead of Button on merit at Silverstone. They've passed Toyota in my opinion and if Ferrari focus fully on the 2010 car, Williams will most likely be best of the rest behind Red Bull and Brawn. There are some tracks coming up that I think Williams and Rosberg will do very well on with the FW31, notably Hungary and Singapore. Valencia and Abu Dhabi could be a similar story, so podiums (and maybe even a win) are not out of the question.
It's just a pity Nakajima falls short, unfortunately i can't see Williams getting 3rd in the constructor's while they only have one driver regularly scoring. 
Nakajima should have scored in the last few races - he's really upped his game. It's just a case of bad luck (and, in one instance, poor strategy) that has prevented him from getting points lately. Hopefully he'll start soon and then Williams can mount their challenge for third place. 
I couldn't agree more. Where as Piquet and Sutil aren't showing development and improvements, Nakajima has come on brilliantly. The 3 last races he's done well. He should've scored in Turkey, bar a botched pit stop. He qualified well in Silverstone, but with such a light fuel load he became a sitting duck. Then, Germany he out-qualified Nico again, only to be hit from behind by Truili, dropping him down to last. But still, he fought back up to 12th with good pace.
However, if Williams do actually take up this option in Hulkenburg, I think they'll be looking at keeping Rosberg as their priority.
Personally, I think Rosberg will be looking to stay at Williams for now. Williams haven't been great in the aero department, but now there's slightly less emphasis on aero, Williams are looking in much better shape. But all the same, Nakajima deserves to stay in F1 after this year.