Wider than shoulder width? From trying to reproduce that movement here in my cube (while glancing around to make sure no one is watching), it seems that if you try a grip that is wider than shoulder width you are creating a mechanical disaster; bi's aren't as focused anymore, and you are stressing the wrists and shoulders.
The widest I ever lift with barbell curls is just at shoulder width.. I would say if you want to go wider you should consider dumbbell curls instead.
* waits for more qualified experts to chime in *