The blogosphere is starting to realize how difficult it would be for the government to "take over" the largest banks, even if those banks are insolvent by various measures. Here is Justin Fox, who considers the size of Citi liabilities relative to FDIC assets. Ezra Klein considers the fate of the progressive agenda if a bank goes on the government's balance sheet and voters start to blame Obama for what they don't like about banks; the email he reproduces is excellent. Matt Yglesias considers the fiscal implications. Plus nationalization can prove contagious. A related issue (I forget where I saw the link) is whether it is legal to nationalize a multinational bank in light of varying national regulations.
That all said, it is an entirely coherent position to wish the government could take over the largest banks.
Here you will find Krugman defending the nationalization idea.