Via Ezra Klein, MIT economist Jonathan Gruber (subject of a CommonWealth Conversation in 2007) says that the Massachusetts health care reform law suggests that national reform can lower insurance premiums:

The major aspects of this reform took place in 2007, notably the introduction of large subsidies for low-income populations, a merged nongroup and small group insurance market, and a mandate on individuals to purchase health insurance. And the results have been an enormous reduction in the cost of nongroup insurance in the state: The average individual premium in the state fell from $8,537 at the end of 2006 to $5,143 in mid-2009, a 40 percent reduction, while the rest of the nation was seeing a 14 percent increase.