For the time being you can use the following CSS workaround:
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style>
.RadForm.rfdButton input[disabled="disabled"].rfdDecorated {
background-position: left -22px !important;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
<asp:Button ID="Button1" Text="text" runat="server" Enabled="false" />
</form>
</body>
</html>