I seem to have the same problem, however it's kind of hard to attach a test case, since I can not really pin down the problem:
In my Grails Project I'm using a JSONP call to get ads for my search results. It works fine in Firefox, camino, but in chrome and safari the script tag is injected in the head and nothing happens. No errors are thrown and no request seems to be made (according to the built in developer tools). The URL is fine and even working, if I copy and paste it to the url-address bar.
The strangest thing is however it's working for me if I use the exact same scripts, etc. in a static html page and even another JSONP call is working fine altogether.
So maybe this might even be a bug in Webkit-browsers and the way they evaluate the script tags? I don't believe that the Grails framework could have anything to do with it, but thats actually the only difference I could see between the two cases.
Browsers tested: Chrome 11.0.696.0, Safari: 5.0.3
Jquery:1.5.0, 1.5.1, 1.4.3
If you need more information I will try to give you as much help as I can...