It can be tedious to write a script tag handling JSNI every time you want to communicate with a trusted third-party server via script tag (unless, of course, you're using jQuery or similar library on the page, but even then the error handling could be a problem). Therefore I created the http://code.google.com/p/gwt-script-rpc/ project to incorporate code for such communication. Using it one can communicate with the server using the script tag by writing just a few lines of Java code, e.g.
P.S. As always, any ideas are welcome here or in the issues list of the project. One area I currently unsure of is how we can help the script tag user to verify if the server he talks with is authentic (before actually trusting it to be the source of a script tag). -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
