Currently, shuttle Atlantis is targeted to launch on mission STS-125 on May 12, 2009. And as NASA lays out an updated schedule, Atlantis will move to her seaside launch pad in April as will shuttle Endeavour as they bot aim for mid-May launches. Endeavour will standdown to Atlantis as she serves as a rescue shuttle if an emergency arises for Atlantis while in space.
Endeavour would then launch in early to mid-June following a successful flight to Hubble by Atlantis.
According to the Johnson Space Center's PAO office this afternoon, the "STS-125 [mission] is an 11-day flight featuring five spacewalks to extend Hubble's life into the next decade by refurbishing and upgrading the telescope with state-of-the-art science instruments and swapping failed hardware.
Scott Altman will command with Gregory C. Johnson serving as pilot. Mission specialists are veteran spacewalkers John Grunsfeld and Mike Massimino, and first-time space fliers Andrew Feustel, Michael Good and Megan McArthur.