But.......does making a powder out of oatmeal turn a otherwise slow digesting carb source into a fast(er) acting carb, in turn negating it's benefits for a bodybuilder?
I don't know that any studies have been done on what you're asking, specifically, but we can reason it out.
Grinding oatmeal decreases the surface area of the oats.
Will this change digestion time?
Yes.
Will the new, faster absorption rate equate to that of simple carbs?
Probably not.
Grinding is a physical change, not a chemical change. You're not altering the molecular properties of the oats, and I doubt you will need to worry about negating its effects. Grinding simply produces a powdered form of complex carbohydrate.
In regards to other posts above, chewing produces both a physical and chemical change. During mastication, the teeth pulverize food, as well the mouth secretes saliva, which contains digestive enzymes, adding them to the food before swallowing. Chewing is the first step in digestion, and a very important one for the reasons mentioned. But, even with chewing, your body will not process oatmeal as fast as it would, say, a candy bar.