As was mentioned in an above post, grain dust will burn, but for it to explode, you need 3 things - particles in suspension, within the right air/fuel mixture range, an enclosed space, and an ignition source.
I can't see the motor clear enough to be sure of what type of enclosure it has - if it's a TEFC (Totally Enclosed Fan Cooled) type motor. It looks like you get some dust in suspension inside your cabinet, but not a terrible amount. You say you had about 1 lb of dust accumulated -- while that might seem like a lot, the key factor is how much of it is suspended at any one time in the area of the ignition source. In other words, if you open the door while running the mill and there's just a light haze of dust inside, there's probably nothing to worry about.
A lot of people don't realize that the big grain mill explosions you read about in the news are actually two- or three-stage explosions. For instance, say a bearing on an elevator leg (not the people kind, instead a large rubber belt with plastic cups to lift the grain, moving at high speeds inside an enclosed shaft) gets hot. While the leg is running, there is a cloud of dust in constant suspension inside the housing. Bearing gets hot enough to become an ignition source, and the dust inside the leg explodes. This is actually a (comparatively) minor explosion, and probably wouldn't even be noticed two blocks away from the mill. But what it does do is shakes the entire building -- and knocks all of the accumulated dust off of the beams and machinery inside. THAT is the suspended dust that ends up blowing the headhouse off the mill and making headlines.
