The possibility is left open that Atlantis may fly one more backup mission after that if necessary, but NASA has not made a decision yet.
NASA said the dates needed to be adjusted because critical payload hardware for STS-133 will not be ready in time to support the planned Sept. 16 launch. With STS-133 moving to November, STS-134 cannot fly as planned, NASA stated so the next available launch window is in February 2011.
The shuttle program has been in service for close to 30 years. After the final mission, US astronauts will fly on Russian Soyuz craft instead.
With the release of NASA 2011 budget and within it the plans to cancel Constellation, Barack Obama made some decisions to make NASA operate differently. The new plan will allow NASA to focus on space exploration while encouraging the commercialization of space.
Check out this interview of Phil Plait discussing NASA’s new budget and why it needs to be pushed through Congress: