It's probably too early to tell how much SARS is being undercounted. We have to remember that a very large percentage of China's population has respiratory ailments of some kind to begin with, so the difficulty in enumerating SARS cases isn't simply a matter of dishonest officials.
It's interesting that SARS infection rate has apparently tapered off in both Hong Kong and southern Guangdong province where it originally came from. In those two places at least, I'm willing to give them the benefit of the doubt, as they're highly integrated with the world economy and major outbreaks are virtually impossible to cover up. If SARS decimates the workforce of some toy factory manufacturing products for an MNC, I wonder how they can prevent that from being known.