Does the problem happen with all Telerik controls or only with certain ones? Does the problem happen on pages where there are no RadControls visible (e.g. only RadScriptManager)? Are you using a <meta> tag to force IE to a specific mode or is it running in the default (IE 8) mode?
The only reference I could find to a 'JSON' keyword in our code is in the jQuery version we are using and even there it is properly checked before it is used. This means that the problem might be caused by another library
the Telerik team