NASA announced that Space Shuttle Endeavour was being moved to the launch pad in preperation for the Hubble repair mission. However, Space Shuttle Atlantis is the primary shuttle for this mission. According to NASA:
Endeavour will be at Launch Pad 39B in the unlikely event a rescue mission is needed during Atlantis' May flight to upgrade the Hubble Space Telescope. After Atlantis is cleared to land, Endeavour will move to Launch Pad 39A in late May for its upcoming STS-127 mission to the International Space Station.
Why the need for a backup shuttle? The threat of a dangerous collision with space junk has increased drastically in the area where Atlantis will be operating. And at this point, the International Space Station will be unable to help in case of a problem.
Picture from marshall43402