According to the Pension Benefit Guaranty Corporation, which regulates and insures pensions, the total deficit in private plans covering about 34 million workers was a little over 10 billion as of September 2008. That's almost certainly multiplied quite a bit since then. But the current underfunding in public plans, which cover about 22 million workers, seems to be something north of a trillion dollars. And they're not insured.
Read the whole thing. When people argue that individual retirement accounts are too risky because human investors are irrational, yada-yada, just remember that government employees who run public pension programs are human beings, too.
UPDATE: see Megan's update. The private sector pension shortfall is larger than what she originally wrote.