IE9: Can't submit node form after opening field settings tab

Steps to reproduce:

1. Use IE9.
2. Add or edit a webform.
3. Add a field, and click on it to open the field settings tab.
4. Try to save the webform.
5. Notice that the form doesn't submit, and none of your changes to the node form have persisted.

When you click on a field to edit its field settings, form_builder does an ajax call to get the entire contents of the field settings form, <form> tags and all. That works fine in form_builder, but in webform_alt_ui, that form is getting plopped right in the middle of the node form. When the node form gets submitted, IE9 sends both form_id parameters in the POST request. (Apparently other browsers are more lenient about the invalid HTML, and only submit the parent form items.) At any rate, form_builder() sees the second form_id, notices it doesn't match the form_id it was expecting, and refuses to process the input.