I looked up a video of ippondori.
When I was watching http://www.youtube.com/watch?v=jWWrCzu-AjA
in the beginning it looked like it would be ikkyo, but the finish is different. Still, to me it seems ippondori and ikkyo are related.
I remember reading that O'Sensei considered ikkyo essential for aikido. Just about every lesson we train ikkyo in some form. Another thing we train just about every lesson is tenkan undo.
(This may all be obvious for others, but I am new)