Yep, Ueshiba said 90% atemi, but he also said other amounts like 70 or 80% (can't remember now). Also Gozo Shioda has said the same thing (which isn't too suprising)- possibly quoted in his 'aikido the master course' or one of the other books attributed to him.
Personally, I don't think the point is that when you do aikido you have to belt them with your fist alot. I think its the awareness (of uke and tori) of all the openings which are presented by the other person in which a strike or kick could be made. Thus if uke is good there are no openings for an atemi and if tori is good the same applies. (The movement of uke to avoid the strike is part of the dynamic motion of the aikido - and obviously if they don't move you strike them).