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

i.e. jQuery with http://jsonip.com/?callback=?

I actually made something like this a while ago.

http://demontunes.com/api/?g=ip // plain ip

http://demontunes.com/api/?g=ip&format=json // in json

http://demontunes.com/api/?g=ip&format=json&callback... // with a callback

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.