The Wall Street Journal has the best map I've seen summarizing where states are on health insurance exchanges right now:

You can also read Louise Radnofsky's and Colleen McCain Nelson's story on the state of play here.

States initially faced today as their deadline to decide whether they would set up the marketplace or leave the task to the federal government. Very late yesterday, the Obama administration pushed that deadline back to Dec. 14, responding to governors' requests for additional planning time.

That did not change the plans in some states: Wisconsin Gov. Scott Walker went ahead and announced this morning that his state would not build an insurance exchange. Most of the other undecided states, however, appear to be laying low.

We still don't know a lot about how these decisions will impact the ultimate rollout of the Affordable Care Act. At the end of the day, there will be a fallback option: All states that don't build an exchange will have the federal government come in and take over the task.

If more states lean on that federal exchange, it creates a larger workload for the federal government. There are already some questions about how prepared the Obama administration is to run an insurance market that could, if states continue to opt-out, cover half the country.