I can't imagine the US nationalising BP. It simply isn't a 'too big to fail' company in America.
Personally, I'd get it out of your investments. There is now talk about suspending the dividend until the oil spill/financial liabilities have been covered. The value of the stock has plummeted since the spill began, and if the (very generous) dividend is suspended, why in the world would an individual/pension fund/etc want to own it?
I think you are right to be worried about the future of the company. Personally, I don't think it can/will survive. Thoughts posted on another thread reproduced here:
I am not so sure that BP will survive this disaster. BP has lost one third of its' market value since the oil spill began.
All the figures quoted (money spent to date, and money projected) to stop the flow of oil doesn't begin to address the company's liability to the businesses/communities/individuals damaged and/or destroyed by the spill. There will be huge environmental costs, not to mention astronomical legal fees. Those figures will dwarf the money spent to stem the flow. If BP is found to be negilgent in the oil spill, the financial costs could (and probably will) crush the company.
And then there is the issue of the coming indictments/criminal charges from the US Federal government - all this results in a liability list that will go on for years, and that even BP, with its $20 billion free cash flow and $12 billion dividend to draw on cannot withstand.