I thought it was one to one (more like, assumed) but I did it with a pound cake and let's just say the product could break a plexiglass window. Is there some ratio to substituting white all purpose flour with whole wheat? are they simply not interchangeable?
Whole wheat flour is denser, has more protein and tends to dry out products. To compensate, sift more, do not overmix, and up your wet ingredients slightly. From eHow. TLC confirms that whole wheat flour is higher in protein because it is milled from hard wheat which is naturally higher in protein, and then ground whole wheat flour is 25% higher in protein than all-purpose flour.
Some sites (including TLC) recommend only subsituting out up to half of white flour with whole wheat, except if you're using white whole wheat flour (see below).
My secret weapon is white whole wheat flour, which is lighter than standard whole wheat flour and tastes more like white flour but has the same health benefits as the whole wheat flour we're used to. Because so many people are looking for whole wheat recipes, you may want to try a recipe that is specifically designed for whole wheat flour--both for taste and texture. Try King Arthur Flour for recipes (they make my favorite white whole wheat flour, and they have a recipe for whole wheat biscotti!).
Weak white flour was mostly invented to make cakes and other soft delicacies, you cannot generally expect wholemeal (whole wheat) flour to work as an exact equivalent in such a thing
Personally the difference is not really important , and is just what you are used too
The protein (gluten) level in wholemeal (whole wheat) depends on wheat source and shelf life. Most of the protein is in the endosperm (the white stuff)
From the same wheat source wholemeal flour will have a lower protein level than white flour due to it being bulked out by the bran (low protein) and germ (no protein), and a lower effect of protein by the effects of the the bran and germ
For kneaded products (bread, pizza dough etc) you can generally replace it 1:1 without any other changes to the recipe, other than a little more kneading. Any difference is generally in what you perceive the finished product should be like - white colour and no fluffy bits (bran)
Wholemeal (whole wheat) has a much shorter shelf life than white flour, This is where most people have unsatisfactory results with it, and it earns it's "hard to use" badge
The simple answer is no, white all-purpose flour is not replaceable with whole wheat flour. You can substitute with it, but the characteristics of the finished product will be entirely different. Your plexiglass smasher sounds about right for using whole wheat flour in a pound cake; it's exactly what I would expect.
Use whole wheat flour when you really want that rich wheat flavor and are okay with the heaviness that comes with it. Depending on the particular whole wheat flour and the recipe you're using, it is possible to make bread with only that, but as Christine noted, it is commonly used to replace only up to half of the white AP or white bread flour in a bread recipe. Even this smaller substitution lends a nice, nuttier flavor to bread.