Synopsis:

This is bad json and will result in an error: { 'foo' : 'bar' } { foo: 'bar' } { foo: "bar" } { "foo" : 'bar' }

You must do this: { "foo" : "bar" }

JSON.parse will also choke on the out-of-spec JSON. If your AJAX interactions stop working with jquery 1.4, it's your AJAX that is broken :)