Well I heard about it in one of the PDC sessions and I try it out. It turned out to be very easy to implement. TollTip tag enables you to set which tag will be host and display as ToolTip. If you choose to use Video tag, you will see the video when your user mouse will be over the control that set the TollTip :
<FlowPanel
xmlns="http://schemas.microsoft.com/2003/xaml"
xmlns:def="Definition"
def:Class="NavigationApplication1.Pane1"
def:CodeBehind="Pane1.xaml.cs"
>
<Button ID="g">click
<Button.ToolTip>
<ToolTip><Video ID="Video" Height="200" Width="200" Source="q.wmv" RepeatDuration="5" /> </ToolTip>
</Button.ToolTip>
</Button>
<SimpleText >Hello World</SimpleText>
</FlowPanel>
