Our cats have fleas again. I've been considering using diatomaceous earth, as was suggested in this thread, but it sounds a bit dangerous to use. Lucas mentioned that it shouldn't be breathed in, but with three cats I can see one of them sniffing another and accidentally breathing it in. Am I worrying about nothing or is this a serious concern?
Dunno how serious it is; I think the warning is mostly for when it's in the air in a big poof of powder, but you probably shouldn't be snorting lines of it either.

Yeah, snorting lines of it would be bad.
Im's right: just don't cake it on so thick that it is able to poof into the air. Basically try to get JUST enough on the skin where normal skin oils and moisture hold it in place. It doesn't take much at all to work: a patch of an inch or two square which is just a layer of one-powder-grain thick diatomaceous earth is plenty. After all, to the fleas, that little bit is like crawling across a field of nothing but barbed wire. [It also kills all eggs and larvae it comes in contact with, which I believe isn't the case with some chemical killers which have to be re-applied as any already-laid eggs hatch.]
And to keep things safest, just like with toxic chemical flea killers, the back of the neck, between/above the shoulder blades, seems best for this, as it is one of the 'least accessible' areas in cleaning, and isn't as often accessed by other cats as other areas might be.
Hope that helps.