Ever since we released the WADL Generator tool (in alpha) for Siena, it's become easy to get Project Siena to work with Salesforce.com.

See http://channel9.msdn.com/Blogs/Microsoft-Project-Siena/App-Tour-Salesforce-Dress-by-Color for an app that delivers a novel fashion selection experience, with the key product data coming from Salesforce.com.

Also see http://www.wadewegner.com/2014/08/build-a-windows-8-app-for-salesforce-with-no-code-using-project-siena/ for how Wade Waggoner built a Siena + Salesforce.com app, including insights about the required WADL.

This is an exciting development, as it opens the window to an even larger number of apps.