I found one article suggesting that 51% of sold housing in Shanghai is vacant (17-Aug-2010). The article also suggests that this is lower than in Beijing and Hainan, but offers no links. Another article (10-Sep-2010) quotes an analyst estimating the vacancy rate for commercial housing stock is between 25 to 30 per cent. And according to this third article (12-Nov-2010), the issue of housing vacancies is being paid special attention in China's latest census.
So the best answer to your question would seem to be a qualified no: while the vacancy rate appears to be quite high, "most" properties in China are not vacant (at the time of writing, and subject to a lack of reliable data).