How do I center a <div> vertically?

Question

How do I center a <div> vertically?
I have managed to horizontally align a Div object.  Here is how I did it.  I got the width and the height of the object I wanted to center (XAML CODE)
[HTML]<Grid  Background="White" Height="300" Width="738" VerticalAlignment="Center" HorizontalAlignment="Center">[/HTML]
Then, in the html file that hosts the silverlight control I did this:
[HTML] <div id="silverlightControlHost" style="width:738px; height: 300px;  margin-top:auto; margin-bottom:auto;  margin-left: auto; text-align:center; margin-right: auto;">[/HTML]
That puts the control centered horizontally at the top of the web page.
Now, how do I center it vertically?