The gov't forced no one to sell in 2008.
Nobody forced that guy to sell that car either. Proving damages is paramount in a lawsuit. Ask a lawyer. One of the first thing he asks a client after explaining what had happened is what are your damages. Selling the shares does not diminish the plaintiffs damages one bit. They had something of value one day; due to defendants actions the next day they lost value. The tort is obvious. Like Barney Frank alluded to; you can't buy something for 10 cents on the dollar and claim you were harmed because you did not get a dollar.
Now, if the government does eventually make things right; let's the senior preferred be redeemed; and releases Fannie and Freddie from conservatorship and relist the shares...then perhaps you have case. the government could argue that they moved to c-ship to protect and preserve assets, and now everything and back to normal so there were no damages. However, I think they would have to do some back tracking to claim that. I think their actions on August 17th were very damaging and constituted a clear "taking" and solidified their September 7th actions.
News 
Market Data 
Discover 
