Drupal 7 AJAX disables my callback I have JavaScript attached to an element in a form that sends it's new value to a PHP callback function. The callback function queries the database and then returns a value that JavaScript then puts in a text box as its value.

I have experimented, and found that callbacks do not work on node edit forms. I assume it has something to do with the new AJAX functionality built into forms.

How to get around this problem?
According to the code example that you posted, you are using a relative path in your AJAX request. Maybe you should try anchoring your request to the base path by using /objnum_callback instead of just objnum_callback. This will ensure that your request is pointing to the same place regardless of the current page.