This is probably fairly obvious to anyone who has been following this litigation carefully. But with the recent scheduling of the leading cases for oral argument in the courts of appeals, it now seems clear--barring a significant, unforeseen event--that the Supreme Court will decide the matter during the October 2011 Term.
The reason is that, with now four cases to be heard by the courts of appeals on an expedited basis in May and June, the decisions from the circuits are likely to come down by the end of the summer. The losing side then has 90 days to file a petition for a writ of certiorari, and the winner 30 days to respond, at which point the petition will be calendared at the Supreme Court. (The loser can accelerate the schedule by filing the cert petition sooner.)
It now seems quite likely that at least one of these cases, and perhaps several, will generate petitions for certiorari soon enough for the Court to resolve them before mid-January 2012. (That is typically the cut-off between Terms; petitions granted after mid-January are typically argued the following autumn.) Thus, the timeline is such that the Court is likely to grant one of the cert petitions in late fall of this year, hear argument in the spring of 2012, and issue a decision by the end of June 2012.
Again, things can change, and this timeline depends on the courts of appeals issuing their decisions in a timely fashion following oral argument. But that seems likely. Which means June 2012--in the midst of the election season--is most likely when the Supreme Court will render its judgment on the ACA.