Is boxed pancake mix usually made out of Allpurpose flour, or cake flour?
Most pancake recipes call for all purpopse flour. I assume boxed mixes do the same, or a mix of all purpose and cake flour for a lighter texture.
The difference between them is 'hard' and 'soft' flours. Hard flours (AP, Bread, semolina, etc.) have more gluten/protein in them, which contributes a chewier texture. Soft flours such as cake are a finer grind and give cakes a crumblier (word?) and lighter texture. If you used AP for an angel food cake, it would be dense and chewy, rather than light and airy.
AP is generally a mix of hard and soft flours, and can be used for many cakes, breads, and of course pancakes. Since pancakes tend to have a chewier texture than many baked goods, they probably use an AP-type flour.
Since pancakes are reletively cheap to make, experiment with it. Try bread, AP, cake, and varioius combinations. There is a recipe in Mark Bittman's How to Cook Everything where you separate the eggs, whip the whites into a foam, and fold it into the batter before cooking. I imagine this would go well with cake flour.