So, the author has said in other forums that it's impossible to inspect the HTTP status code of an iframe via javascript. So, I developed a work-around in Rails.

In my controller, I have something like this:
respond_to do |format|
if @object.save
format.js { render :partial => "success_partial", :status => :created }
else
format.js { render :json => @object.errors, :status => :unprocessable_entity }
end
end

Notice if the save succeeds without errors, the app returns the HTML that's in the success_partial along with a successful :created (201) HTTP status code. If the save fails with errors, the app returns a JSON hash of the error messages with a 422 status code (which the jquery form plugin can't see because it goes to the iframe and can't be recovered).

So, in my javascript response-handling code (this is for Rails 3 using the rails.js jQuery adapter), I test the actual response and manually trigger the ajax:failure if the response is able to be parsed as JSON:

$("#my-form")
  .bind("ajax:success", function(evt, data, status, xhr){
    try{
      $.parseJSON(data); // throws error if data is not JSON (meaning success)
      $(this).trigger('ajax:failure', xhr, status, data) // only gets to here if JSON parsing was successful (meaning data is error messages)
    }catch(err){
      // do nothing
    }
    // do some success handling
  })
  .bind("ajax:failure", function(evt, xhr, status, error){
    // do some error handling
  })

Hi Steve,

This is pretty much what we ended up doing: returning something that signalled an error in a successful response, which we then parsed and handled accordingly.

Cheers!