In Drupal 6's excellent SimpleTest module, a method called drupalPost() allows you to simulate a button press on a form by taking the form's data and using HTTP POST to submit it. But what if you want to POST data to an AJAX callback URL? By default, SimpleTest checks which submit button you have pressed, but of course, when POSTing using AJAX, you probably won't have pressed a button!
After doing some searching on a problem he had - submitting a form without the actual form on a page - he decided the best solution was to create a base class that inherits from DrupalWebTestCase. This base class allowed him to make a POST request (via curl) to the page and simulate a form request. The code for the method is included.