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. }

So far I have found two examples of a stand alone strategy for Silverlight.

Silverlight: Running standalone full trust applications

How to make Silverlight be AiR?

Both pretty much use the same method by employing MSHTA with an *.hta file.(also achieving full trust for the application)

The trouble is these solutions are not a cross platform answer, and I don't see the benefit this would bring over a WPF click once app or an xbap application.  At least its possible.  Maybe Microsoft will find it beneficial to provide an attractive offline mode for Silverlight apps.