Adding a general parameter to all ajax calls made with jQuery

What you're looking for is a Prefilter, here's a sample from the page:

$.ajaxPrefilter( function( options, originalOptions, jqXHR ) {
  // Modify options, control originalOptions, store jqXHR, etc
});

This requires JQuery 1.5.

You could use the $.ajaxSetup method as illustrated in the following article.