A more gentle transition
There has been much gnashing of teeth concerning the imminent, and all but inevitable, gap in American manned spaceflight capability after the Shuttles are mothballed in 2010. I've been thinking about this problem, off and on, for some time now. Given the current state of NASA and the commercial space transportation industry, there are really only a couple of realistic options for dealing with "the Gap".
The first option is for NASA to continue flying the shuttle a couple of times a year beyond 2010 until a suitable replacement is available. I have never really had a problem with flying the Shuttle past 2010. It will be just as dangerous to fly it then as it is now (and yes, I would jump at the chance to fly in it if it were offered to me). Many have dismissed the possibility of extending the Shuttle service on the grounds that: a) it's too expensive and would consume resources that NASA would rather be devoting to Ares I/Orion, and b) it wouldn't solve the underlying problem of how to get our own astronauts to and from the space station without having to rely on the Soyuz capsules as lifeboats.
In my own opinion, I think that using the Shuttle strictly for crew-rotation and logistics deliveries is a terrible waste of the Shuttle's unique capabilities. Instead, I'd like to see NASA take advantage of these extra flight opportunities to continue expanding the ISS beyond the minimal 'US core complete' configuration. Many additional modules have been, or are nearly, completed. The Shuttle was conceived, designed and built to support the construction of a Space Station. So long as the Shuttle is still active, it should be doing what it does best.
Another possibility for reducing 'the Gap' is for NASA to provide additional resources to companies like SpaceX to assist with the rapid development of launch vehicles suitable for manned spaceflight. For a very small fraction of the resources currently being poured into the Ares/Orion development, SpaceX could potentially have the Falcon 9 rocket and the Dragon capsule ready two to three years before the first Orion capsules would be available.
There may other options which have some chance of mitigating the consequences of retiring the Shuttle before its successor comes online, but for now, I'd like to explore a third option that lies somewhere in between the two options mentioned above.
Let's assume that SpaceX will be able to demonstrate reliable cargo delivery to the station and safe return to the Earth with the Dragon capsule by the end of 2010 or early 2011. At that point, the main thing preventing crew rotation using the Dragon is the development and demonstration of safe and reliable crew launch on the Falcon 9. Elon Musk has stated that SpaceX could have crewed Dragon ready within three years if they were to be funded under the COTS-D program (perhaps a couple more years if no COTS-D funding were provided). That's most likely an optimistic estimate, but even still, that means no crew launch capability until at least the 2012-2013 time frame.
Let's imagine then that NASA is persuaded by Congress to continue flying the space shuttle to the station twice a year until a suitable replacement vehicle is ready. By that time the Dragon will likely have demonstrated the ability to stay on orbit at the station for months at a time and then execute a safe and controlled reentry. With probably very few modifications, it should be possible to convert the Dragon capsule into a crew life boat once it has delivered its cargo to the station. That's a lifeboat that can seat seven (i.e. the entire ISS crew complement with room to spare).
So long as no problems arise, the crews can be rotated in and out on the shuttle. The Dragon could continue to be used to de-orbit important cargo, but in the event of a emergency situation, it could also be used to return all or just part of the crew. Assuming the Russians still have at least one Soyuz docked to the station, the redundant life-boat capability would mean that the station would not necessarily have to be abandoned if only one or two of the crew needed to be immediately returned to Earth.
So, rather than saying either Dragon or the Shuttle, why not say both. We can make the most out of the Shuttles' extension to really finish building out the ISS. At the same time, the Dragon can be fulfilling a useful role as a lifeboat for the expanded crew without having to rely on the availability of additional Soyuz capsules. When the Falcon 9 / Dragon capsule has been qualified for manned launches, then crew rotations can be transitioned from the Shuttle to the Dragon, and the Shuttles can finally be retired for good.