The WAI-ARIA specification indicates that there are alert, alertdialog and dialog roles, the appropriate one must be applied to the RadWindow dialogs.
http://www.w3.org/TR/wai-aria/roles#alert
http://www.w3.org/TR/wai-aria/roles#alertdialog
http://www.w3.org/TR/wai-aria/roles#dialog

In the meantime, you can use a script like this to apply the role:
        <script type="text/javascript">
            function OnClientShow(sender, args) {
                var role = "dialog";
                 if (sender._isPredefined) { //this captures RadAlert, RadConfirm and RadPrompt
                   role = "alertdialog"; 
                }
                //this will be executed for all child RadWIndows from that manager so you can apply a different role (e.g., dialog)
               sender.get_popupElement().setAttribute("role", role);
            }
        </script>

        <telerik:RadWindowManager runat="server" ID="RadWindowManager1" EnableAriaSupport="true" OnClientShow="OnClientShow"></telerik:RadWindowManager>


Note: Testing with the JAWS screen reader indicates that it does not recognize dynamically created HTML, and all RadWindow dialogs are created dynamically with JavaScript. This is a shortcoming of the reader.