AJAX file upload is broken

I've tried attaching a file to a node with JS enabled using both Firefox 1.5 and IE 6. With both browsers I see the same result.

"Uploading file" with a progress graphic. The browser throbber animates for a bit and then stops but the upload fieldset never changes.

Clicking 'submit' on the node shows that no file was attached and no file is visible in the Drupal filestore in either the files/tmp directory or elsewhere.