Grain dust explosions can be dangerous. Fine grain dust will suspend in the air quite easily and is a bomb waiting to happen. All it takes is a spark. The incidence of grain elevator explosions has been dropping in the last several years due to more stringent safety standards, but they still happen. There was one in Kansas just a couple of years ago that killed half a dozen people and took out most of the windows in town. Here is a link to the picture of that.
http://www.lincoln.ne.gov/city/fire/usar/pdebruce.htm
On the homebrew level, we don't have much to worry about. My system keeps the dust saturated air away from my drill so there is not much chance of spark ignition. Basically, if the dust in the air is not thick enough to cause a little bit of irritation when you breathe, you are probably safe.
Wayne
Bugeater Brewing Company