What are the differences in bleached/unbleached flour for baking?
Good Answer Hobodave....tacking onto that:

Most southern U.S. brands (White Lily, Martha White) of all-purpose flour are bleached because southerners tend to make more quick breads (biscuits, cornbread, hoecakes, pancakes, as well as cakes, pie crusts, cobblers) where tenderness is desired.  White Lily has just started producing unbleached all-purpose flour as well.

Brands from the Northeast (King Arthur) and midwest are usually unbleached because those regions tend to make more yeast breads and the additional gluten forming capability of unbleached flour produces better results.

National U.S. brands (Pillsbury/Gold Medal) often produce both types or stick with unbleached.

Cake flour is always bleached (usually chlorine gas).

More information on differences in types of flour can be found in my article titled "Flour Power"