I just read this, which is a letter from McCain and two other senators to the President concerning the Shuttle, ISS and the new programs.
Basically, they recommend that NASA not do anything that would prevent the Shuttle from operating beyond 2010. They basically argue that the USA must not lose it's human spaceflight capability at any time, meaning the Shuttle cannot be scrapped until a replacement is available.
Sounds like a good idea, though it is based on the political reality that Russia may get mad at us. The details of that are for another forum.
I've been wondering about the best way to keep the Shuttle "simmering" though. No actual flights is a big waste of money for systems upkeep. To many flights raise safety concerns. The purpose of the missions is also an issue. ISS flights are obvious, but what about pure scientific flights?
Sometimes I wonder if we aren't killing the bird in it's prime.