It looks like the loading graphic is being shown onLoad, and the script tries to figure out the content type before this point. So if you have a slow web service, it may take time to realize the content type, thus not showing the loading.gif.

I did a quick test, and displaying the gif onOpen seems to work:

$(".myClass").colorbox({
   onOpen:function(){
       // taken from colorbox.css 
       $('#cboxLoadingGraphic').show(); 
    },
    onComplete:function(){ 
       $('#cboxLoadingGraphic').hide(); 
    }
});


Jquery - Customizing the colorbox plugin
I can't think of the word to describe it. Something similar to "wanderer" or "roamer". 

It's often used to describe people that don't stay in one place... not "migratory"...
