Jim,

My understanding is that it is a human-health issue (as opposed to, say, yeast health or dough vitality.)

Bleaching of flour involves adding some sort of...well, additive (of the chemical persuasion) to the flour, basically to make it whiter ("more appealing," the bleach folks will say, since apparently (???) yellow-tinged flour/bread is "unattractive" for some reason...). Depending on the type of bleaching agent used (benzoyl peroxide and -- gasp -- chlorine are both commonly used), nutrients (including Vitamin E) are lost from the flour, and human health may be affected. Certain types of bleaching agents have been linked to diabetes.

Also, many bleaching agents produce chemical by-products (again, think chlorine) that are environmental toxins that can leach into ground water and cause all sorts of problems.