I agree. Let's stop for a moment - by atemi, do you mean actual striking? I ask because there are styles of Aikido that don't actually strike, or much anyway, that come from very close students of O Sensei. Would you say they are not doing Aikido?
Difficult to answer in writing, I am not talking about karate or boxing like strikes peppering ones waza. I think that atemi can take on various forms, from maintaining a position where one could strike uke the moment a suki develops in the relationship (as George described) to positional strikes (like those seen from Nishio Sensei' irimi movements) to impacting movements through the uke/nage connection (like you see in videos of Akuzawa or Saito Senseis for that matter) or even movements that seem like blocks but are actually strikes (Don Angier's 'cam' for example). It can also be that the mechanics of striking are used in the movements themselves: an extension of the arms to do kotegaeshi, the cut of the arms in shihonage, a rake to the eyes when setting up for shihonage (a la Nishio Sensei again), or the impact of ones koshi into uke's kua line when performing koshinage/o-goshi. I think this is what OSensei meant when he said 90% of Aikido is atemi, and when Tomiki Sensei said that, "...the striking techniques of Aikido incorporate the idea of balance breaking; the result being that the opponent is brought down due to loss of balance rather than because he was hit on some vital point. Thus it is not necesary to kill or hurt him by using strong impact, nor is it necessary to train your hand or fist to withstand such impact." When I talk to my former Aikido dojo-mates and explain that where I train now, we actually land our atemi most of the time, they think that we're punching each other in the face all the time. This isn't true, but we are looking at how atemi within the context of an aiki art is actually used to get kuzushi and affect body structure just as Tomiki described. (Note, that I'm not part of a Tomiki lineage, I do really like his writings on budo.)