NASA head Mike Griffin has said that they will go and fix Hubble in September 2008. Service Mission Four (SM4) will not leave for Hubble any earlier than August 2008 but could be later than September if it means a better result in the end. Either way the mission could extend Hubble’s life to 2015.
In January 2007 the ACS intsrument failed and this followed the September 2004 failure of the highly valuable STIS. Both of these could be repaired if the mission goes along current plans. In addition, NASA are looking to add Wide Filed Camera 3, to replace the ageing WFPC2 and also add several other vital astrophysical instuments which will no doubt create invaluable data for scientists worldwide.
Gyroscopes, batteries and other parts of the space telescope need repairing or replacing and all of this will require a very long mission, incorporating possibly 6 or 7 EVAs (missions outside the shuttle). The mission is also dependent on the continued safe operation of the shuttle fleet and also on the condition that during SM4 a second shuttle is always prepped for launch in case the astronauts need assistance, since Hubble will be too far from the ISS safe haven.
If this all works though, and Martin Barstow seems to think it will, then Hubble will be operating better than it ever has done and will provide more science than ever before.