changing site.master page dynamically based on url

I have a website say http://www.xyz.com. Now I also want to have this same website as an app in facebook say http://apps.facebook.com/xyz.

The only difference will be that in开发者_StackOverflow my site I want to have my links on the right side of page vertically but in my facebook page I want to to have the links in a horizontal bar.

As the website is the same it would be best if I could have two master pages and call them based on the url.

Any idea if this is possible.

Thanks Arnab