The SC has decided to hear a case to determine if the federal government can extend health care subsidies to citizens in the states that have not set up their own marketplace and instead use the federal marketplace.
According to Lyle Denniston,
By adding the case to its decision docket at this point, without waiting for further action in lower federal courts, as the Obama administration had asked, the Court assured that it would rule on the case during the current Term. If it decides to limit the subsidies to the state-run “exchanges,” it is widely understood that that outcome would crash the Affordable Care Act’s carefully balanced economic arrangements.
Obviously the drafters of the legislation intended for subsidies to be available to participants in the federal marketplace. You can ask them and they'll say so. To any unbiased observer that is clearly what was intended. However the loose language may allow the ideologues on the SC to unravel an important part of Obamacare.