The Court will consider tomorrow whether the president can indefinitely detain, without criminal charges, a U.S. citizen captured on American soil in the so-called War on Terror. Jose Padilla is the U.S. citizen who supposedly plotted to detonate a "dirty bomb." Since his capture at Chicago's O'Hare Airport he has not been charged with any crime. Earlier this year a federal judge ordered the government to charge Padilla with a crime or release him within 45 days. It is this ruling that the government is appealing.
A news report on Padilla can be found here.