I visited Beijing early Feb. 2005. I don't handle the cold very well and felt *really* cold at the Summer Palace with the snow, wind, and ice. On the plus side, there were very few tourists around in general.

Also, try to plan around Chinese New Year. It begins on January 26, 2009 and involves two weeks of festivities. The Chinese people get that time off for the celebration, so there will be crowds everywhere during that period.