"NASA managers have announced that they will not meet a February 2009 launch date for the fifth and final shuttle mission to the Hubble Space Telescope."
The ground spare to replace a defective unit aboard Hubble will not be ready to support a February 2009 launch so a new launch date is needed. A significant anomaly occurred during testing. Six and a half months of testing is needed before the unit can fly. NASA's plan is to have the spare unit ready to ship in the April 2009 timeframe so as to support a May 2009 launch.
This will also cause an additional delay of Ares 1-X