Many of us were awaiting the STS-133 launch November 1 from Florida. Since then the flight has been grounded by cracks in stringers supporting the External Tank. The legendary Flight Controller and JSC Director Chris Kraft suggested that "material properties" were to blame. The current JSC Director reports that the problem has been traced to a single batch of stringers produced in 2002. The stringers were not made too weak, but unusually rigid. As such, they had a tendency to crack under stress rather than bend. NASA engineers are currently working the problem, still hoping for a launch February 24.
Though Congress has not yet appropriated the funds, JSC is still working on STS-135 with a tentative launch date of June 28. This or STS-134 could be the last flight of the Space Shuttle.