Air quality in Beijing became the central concern these days. Asia Society put together this website offering a visual and statistical perspective of how it is like.
P.S. Yes it is hot today in Beijing and my colleagues asked me why they picked hot summer days for the Game. Superstition rules (8pm, 8/8/08) was their hypothesis. But my guess is that there are spring sandstorms and autumn fogs in Beijing, and winter is not pleasant enough for any outdoor event. So summer is our best bet.