You should have probably noticed that when developing Metro apps for Win8, the WinRT currently doesn't include any access to embedded relational database such as SQL CE. However there's a light at the end of tonnel in the form of the SQLLite - an open source embedded database that you should be able to use in your Metro apps. Checkout this blog post by Tim Heuer:

http://timheuer.com/blog/archive/2012/05/20/using-sqlite-in-metro-style-app.aspx