NASA has announced the new target launch date for the fifth (although it's called Servicing Mission 4) and final mission to service the Hubble Space Telescope. Atlantis will launch for STS-125 on October 8 of this year, if all goes well. The mission was previously delayed due to NASA's need to build fuel tanks and for Endeavour to get ready. Endeavour is necessary as a back-up if a problem happens with Atlantis, as the shuttles trajectory to Hubble will make it impossible to make an emergency trip to the ISS.
If everything goes well and Endeavour is not needed for a rescue mission, then that shuttle will fly on November 10, 2008, to deliver a Multi-Purpose Logistics Module (MPLM) to the ISS.
When Servicing Mission 4 is complete, not only will Hubble's useful life be extended, but the quality of science it can achieve will be enhanced. These enhancements include new batteries to replace the batteries that have been in use since 1990 and replacement of some of the gyroscopes that keep Hubble pointed at its targets for observation.
The crew will also be repairing the Space Telescope Imaging Spectrograph and some of Hubble's insulation, as well as installing a new cooling system, the Wide Field Camera 3, and the Cosmic Origins Spectrograph.