Here is how things look so far:
BMW, Mercedes, Renault, and Toyota have all announced that they are still in.
Honda is out.
That leaves Ferrari, Red Bull/Torro Rosso, Williams, and Force Inda as "undeclared".
Ferrari is easy. Unless there is some serious schism with the FIA, Ferrari will be in. History shows that the schism would have to be pretty severe, as Ferrari and the FIA have always found a way to live together in the past.
Red Bull/Torro Rosso is also pretty easy. While it may be premature to call this one, I will predict that Red Bull will try to sell Torro Rosso over the winter and then shut it down at some point through the 2009 season when such a sale proves to be impossible in the current environment. The parent team is good through 2009, but if things don't turn around both on the track and in the economy at large it too is at risk for 2010.
Williams is harder to predict. On paper they are vulnerable. They are tied to another manufacturer in Toyota, who may or may not find it cost-effective to continue to provide engines. Their long term sponsorship deals are all with companies which will be hurt in the downturn. However Frank Williams and company are very resourceful and have been written off before. This one is too close to call.
And finally Force India. I really don't know much about the ownership structure, or the culture of ownership of the team. "Indian Billionaire" is such an odd description for a team owner. I can't say if the ownership is capable of weathering such an economic storm.
So in conclusion: Honda gets shut down over the winter; Torro Rosso gets shut down before the end of 2009. And next winter we get to look at Red Bull and Williams as the weak links if the economic climate doesn't improve.