It looks like there will be a national exchange that a resident of a state who did not implement the exchange can join. States can challenge the Federal Exchanges in Court but its unlikely they will prevail..
in WI, Gov. Walker stated that if the state started an exchange, they’d be stuck paying for it. If the feds do so, they’ll have to use federal money. He says that since a majority of people don’t want Obamacare, it would be wrong to force his state to waste it’s money on it. Let the feds spread the wealth around.