I have just updated to JQuery 1.5 and all my ajax calls that return JSON and a number of plugins instantly broke.
In my pre-1.5 code, I specified the dataType like:<
I came across this JQuery bug which fixes the problem http://bugs.jquery.com/ticket/8084.
Adding the following code after the JQuery script declaration fixes the problem:
$.ajaxSetup({ jsonp: null, jsonpCallback: null });