You should probably include the ?callback= functionality anyway. A great deal of libraries assume it will be a get variable, and cater for it specially
your absolutely right, but I did not make jsonip.com
If you are using a framework for this, most of the time you are able to override callback function name, i.e. jQuery uses jsonpCallback : http://api.jquery.com/jQuery.ajax/
If not, load it manually as a script and generate a function for the callback.
i.e. jQuery with http://jsonip.com/?callback=?