Over the last 9 or so months I've looked at many Ajax examples. Hundreds. Recently it suddenly dawned on me that designing for Ajax is to some extent to forget the submit button. You still need forms to create input fields and such, but you don't need to press a submit button to pass form data to the server. It could as well be a clicking on a link, or an onchange, onblur or onkeyup, or another intrinsic event. As a test, remove the submit button from one of your forms and see what happens! Your mind just might go into Ajax mode there and then!
But you can just as well look at the whole issue the other way around: when a user clicks on a button, it doesn't have to be a form submit! It can be an Ajax call to do something else. A really good example is the "menubutton" class used by Yahoo mail. Clicking the "Move..." button brings up a dropdown menu with all the folders where you might want to move your email. It doesn't have to be implemented as an Ajax call (I'm not even sure it is), but it could be.