Fine, this is mostly just conjecture, but here goes...
Based on the data trends prior to the cheating that started may 10th, I'd guess China now has roughly 8400 'probable' SARS cases instead of the 5300 they claim.
But two things make me feel it is probably worse than that...
Just before May 10th, I can see that SARS had gotten a small but definate foothold in 4 or 5 MORE provinces. Not enough so as to dent the overall numbers, but noticable. Those outbreaks have probably grown considerably, which would add even more to my 'guesstimate'.
Secondly, this thread suggests HOW and WHY the abrupt data shift occurred. I believe this story about the Beiyuan Gardens could be an example of the new 'norm' in China, and not an exception to the rule.
Possibly, starting May 10th or 14th, residents with fevers were told to stay home instead of hospitalization. (China's healthcare system had presumably overloaded and shut down). We can surmise this from reports of hospital workers getting sick themselves, others (doctors and nurses) for a while shoving the work onto the new interns/residents, and yet others outright quitting.
I doubt right now if China cares if residents expire in their housing projects like Beiyuan Gardens, just so long as they remain there and don't spread it to outsiders. That would be their reason for installing thermal sensors at all the exits of the complex. They are essentially isolating the sick within that complex instead of in a hospital.
This poorer method of isolation would also lead to higher infections. But none of these people are being recorded as SARS patients, and if they die aren't being being recorded as SARS deaths.
Maybe China has decided to just contain this as best they can, and see if it will burn itself out.