NASA has decided to postpone the launch of Atlantis once more, due to work being done on the Engine Cutoff Sensors, which have been giving spurious results. The ECOs report on the amount of fuel left in the tank, and are designed to cut off the engines if the fuel runs too low.
The ECOs Hardware associated with the ECOs has been pulled out and is being worked on. NASA has not said when the Shuttle will launch again, or how this slip will affect the already overpacked schedule for the next few flights. We'll find out soon.
As usual, Damaris has the inside scoop.