The date is set. Atlantis will fly the 135th and last mission of the Space Shuttle era on July 8, taking a load of new components, spare parts, and other equipment up to the ISS.
From the NASA press release:
Atlantis' STS-135 mission will deliver the Raffaello multi-purpose logistics module filled with supplies and spare parts to sustain space station operations after the shuttles are retired. The mission also will fly the Robotic Refueling Mission (RRM), an experiment designed to demonstrate and test the tools, technologies and techniques needed to robotically refuel satellites in space - even satellites not designed to be serviced.
COMMENT: The refueling experiment is pretty interesting. Some aspects of on-orbit servicing have been tested, but this will advance the state of the art considerably. It's an old debate whether it's worth developing an operational on-orbit servicing capability. I think it's worth continued testing.