Using AJAX in Drupal 7 to change a form action
For more examples, here is the link to Lullabot's jQuery and JavaScript in Drupal video tutorial source files. I would pay attention to the Drupal 7 quickview.module example.

Here they use a page callback that passes either 'nojs' or 'ajax' as the second argument. The Drupal ajax framework will change the links on the page from 'node/[id]/nojs' to 'node/[id]/ajax' if the client has ajax enabled. The page callback function tests for the presence of ajax. If not, it will use drupal_goto() to gracefully degrade and reload the page. Or, if the ajax framework changed the link to ajax it will construct the HTML can send it back to the page with json headers and all using the new command() to manipulate markup, javascript Drupal setting, or CSS on the page.

I image with a tweaking this same technique can be used for Form API.