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Why are you using jQueryXX.vsdocs.js files?
The issue was actually with the jquery-1.5.1-vsdocs.js JavaScript file. For some reason, it was throwing the above error in the $.ajax() call. Once I took the script reference out, and it didn't get loaded the AJAX request worked.
Any ideas or theories as to why this caused the issue would be much appreciated. But the removal of it fixed the code.