Just a reminder to myself, so I won’t waste as much time if it happens again.
When a jQuery.getJSON() callback is called by Firefox but not by Internet Explorer, check if the JSON data returned really is squeaky-clean.
An extra comma at the end of an object declaration was doing okay with the more forgiving Firefox JSON parser, but tripped IE’s, with no error message, and no callback ever called.
I found the error by using jQuery.load() instead and eval()-ing the AJAX responseText in the callback, which threw a JavaScript error this time.