With the validation plugin (http://bassistance.de/jquery-plugins/ jquery-plugin-validation/) I included two "remote" rules to check username and e-mail. If my form is loaded such that the username and e- mail are already filled in, and, therefore, have not yet been validated, and I click SUBMIT, validation proceeds just fine, but the form does NOT submit! A second click of the submit button, however, then works.
I speculated that this was because of pending ajax calls aborting the submit. It seems my speculation held true. Right after my "$ (document).ready(function() { ..." I added the code: $.ajaxSetup( { async: false } ); to make the "remote" ajax calls synchronous and this fixed the problem. Now clicking submit performs the validation and really does submit. Hopefully a more elegant solution will be found in the validation plugin itself, but as a jquery newbie, I found reading the validation plugin code daunting!