Joel Cochran explains how you can center your Silverlight App in a web page.
To accomplish this little trick I need to define a wrapper DIV in my CSS. This Div will then wrap the entire contents of the site, so now I only have to center that DIV. We need to define the width of the content area: it makes sense that you want to set the size of something you want to center so it can be calculated. To do this, I set the width property of the wrapper DIV to the pixel width of my Silverlight control.