Burp plugin for scanning GWT and JSON HTTP requests
Update: Burp Suite Pro 1.4.10 supports JSON scanning out of the box, see http://releases.portswigger.net/2012/06/v1410.html
Update 2: The plugin is released under the terms of GNU GPL. In short it means that you can use it and change it as you like, publish the changes under GNU GPL if you like, but cannot include it as a part of any closed-source software. If you really want to use it as a part of closed-source software, contact me, we can figure something out.