According to client reports, having an ID set to the HTML element that holds the text of the predefined dialogs improves the way JAWS handles them. Here is a small example that shows how you can do that:

<telerik:RadWindowManager ID="RadWindowManager1" runat="server" OnClientShow="OnClientShow"></telerik:RadWindowManager>
<script>
	function OnClientShow(sender, args) {
		if (sender._isPredefined) {
			var dialogId = sender.get_id();
			var textContainer = $telerik.$(".rwDialogText", sender.get_contentElement());
                        //for the Lightweight RenderMode, use the ".rwDialogMessage" selector
			var textConainerIdPrefix = "dialogContent_";
			textContainer.attr("id", textConainerIdPrefix + dialogId);
		}
	}
	function pageLoad() {
		radalert("a warning");
		radconfirm("a confirmation");
		radprompt("a prompt");
	}
</script>