the SARS coronavirus is undergoing rapid evolution in our populationThis is entirely typical coronavirus biology. That's why there are hundreds (or more) cold viruses, and that's why there is no cold vaccine.
If the SARS coronavirus acts like other coronaviruses, it will spread rapidly, become endemic, and change constantly.
The mutants are good. The deadlier forms tend to disable and kill their hosts too quickly and so they don't spread, the milder forms let their hosts survive and expose more people. Getting a milder form gives you the best protection, the most cross-immunity. Vaccines don't work as well as getting a full blown but mild disease.