ASP.NET MVC 2.0 will now, by default, throw an exception when an action attempts to return JSON in response to a GET request. I know this can be overridden on a method by method
I also got this error when I first use MVC 2.0 using my old code in MVC 1.0. I use fiddler to identify the cause of the error. See the steps on how to troubleshoot it using Fidder -