I frankly don't see how Aikido exists in absence of atemi. I strongly recommend that people read George Sensei's column on Atemi:
http://www.aikiweb.com/columns/gledyard/2004_08.html
Chris makes a very good point that many people use an atemi as an attempt to compensate for the failure to unbalance the person (which an atemi is an indispensable component of).
marc abrams