From the Financial Times:
Central banks float rescue ideas Central banks on both sides of the Atlantic are actively engaged in discussions about the feasibility of mass purchases of mortgage-backed securities as a possible solution to the credit crisis.
...
Any move to buy mortgage-backed securities would require government involvement because taxpayers would be assuming credit risk.
To understand the economic arugment, see Professor DeLong's
Post-Meltdown After-Action Report: The Housing Market, the Tasks of the Fed and the Treasury, and Economists' Reputational Bets (see diagrams about half way down post).