Just remember if you run into a control that does not support ToolTipService(). ToolTip you can always create a invisible boarder around the control.  And you can use that border to display your tool tip.

From the channel nine screen cast Silverlight - Handling HTML Events in .NET Code by Mike Taulty

Void OnLoaded (object sender, RoutedEventArgs e)
{
    HtmlElement element =
        HtmlPage.Document.GetElementById("MyButton");
    element.AttachEvent("onclick", (EventHandler)MyHandler);
}
void MyHandler( object sender, EventArgs args)
{
    //Do something.
}