George Ledyard Sensei had written an article on Atemi. It is an excellent article that goes into some detail. My teacher, Imaizumi Sensei, stated about six months ago that if you move properly, technique is frequently not necessary. He was directly referring to moving in unison with an atemi.
Imaizumi Sensei and Nishio Sensei saw eye to eye on the "use" of Atemi. As Nishio Sensei once said Aikido is practiced with the rythem and flow
of Atemi. Proper movement/footwork is the key.
On another note I disagree with Ellis's comment about about a Martial Art (as posted by George) and it sounds like he may have been trying to make a distinction between the Koryu and Gendai Arts which in that context makes sense to me.
Aikido is either a Martial Art or it is not and I am ok in either case with whatever anyone chooses to call it. I know what it is to me.