See here. A ruling from the three-judge court by mid-August would give just enough time for some emergency action at SCOTUS before the election through some kind of emergency stay or injunction. This would be coming while the Justices are scattered before the beginning of the October 2012 term.

DOJ wants the issue resolved in the Shelby County case first.  Wonder if that could be expedited to SCOTUS as some way of dealing with the constitutional issue in a case where there is already a substantial record developed on the issues.