Both shuttles (Atlantis on Launch Pad A in the foreground, Endeavor on Launch Pad B in the background) are on the pads for the Hubble servicing mission slated for October 10, 2008. When the ISS is not available for rescue purposes, as it might not be for this mission, a second shuttle is made ready for a quick launch. Tom’s Astronomy Blog has some additional detail.

The last time such an image could be seen was July 2001, for the STS-104 and STS-105 missions. Unfortunately, we’re not like to see it a 3rd time, as the shuttle fleet is being retired in 2010.

[NASA, Tom's Astronomy Blog]