What Mary said. It's dust. Mineral dust, at that. Is mineral dust good for you to breathe? Of course not. THat doesn't mean we should freak out about occasional exposures, people survive occasional dustiness just fine (assuming no underlying respiratory disease), but it does mean you should try *not* to be breathing it, when possible. It's real easy to just put a mask on when you apply it, and there ya go.
I do not personally believe it is a problem for chickens WHEN USED INTELLIGENTLY (as opposed to slathered thickly all over the coop on a frequent basis, as a few BYCers sound like they're doing)... at least, the way I use DE, a little bit under the bedding, it does not seem to get 'kicked up' as dust and you don't hear about DE-fiends' chickens dying of respiratory disease at an obviously accelerated rate.
So, just use some common sense and avoid snorkin' it into your lungs when applying, and IMO you will be fine.
Good luck, have fun,
Pat