Hello Kenichi,

This is available in WPF. To enable this feature, you can use the WPF native ToolTipService and its ShowOnDisabled attached property. Note that in this case you will need to use the native ToolTipService, instead of Telerik's RadToolTipService. Also, keep in mind that the Telerik themes have styles for native ToolTip so it is going to be consistent with the other parts of the Telerik UI.

<telerik:RadButton ToolTipService.ShowOnDisabled="True" ToolTip="Tooltip content" IsEnabled="False" />

Regards,
Martin Ivanov
Progress Telerik

The XAML team has recently reviewed this issue and will not be addressing it due to Silverlight framework limitations. This is dictated by the fact that, it is not possible to track mouse events on a disabled element. If you want to achieve the same behavior in WPF, you can use the native TooltipService. The native ToolTip supports  ShowOnDisabled functionality and Telerik provides theming support for native controls to achieve consistent UI with Telerik suite.