NASA has posted its Space Launch System Acquisition Overview, the first step towards development of the SLS design released on September 14.
According to the post:
The Space Launch System, or SLS, will be designed to carry NASA’s Orion Multi-Purpose Crew Vehicle, as well as important cargo, equipment and science experiments to Earth's orbit and destinations beyond. Additionally, the SLS will serve as a back up for commercial and international partner transportation services to the International Space Station.
I can't recall anyone other than politicians insisting that SLS is a "back up" for ISS access. SLS is designed as a heavy-lift vehicle for travel beyond Earth orbit.
The overview states that existing contracts for the cancelled Ares program will be used to develop the SLS.
UPDATE September 24, 2011 — Florida Today reports that the SLS contractors will be those who worked on Constellation and Shuttle.
Just more proof that Space Launch System is a politically mandated jobs program.