Stay inside today; Beijing's API currently stands at 268 (PM10 concentration 397 ug/m^3). This is considered Very Unhealthy. (More info here.)
I thought this was a dust storm blowing into Beijing, but now I can't find any media references to one outside of this article from Sunday. (FYI, Sunday's API peaked at 119).
This is the second day this year with an API above 200. The first was the day of the Mandarin Oriental fire.
Update 3/19/09: Check out this terrific NASA image from Sunday:
Again, this was Sunday, whose API was 119. Still no word on the cause of yesterday's 268.