For most businesses it probably makes the most sense to write a tradtional HTML5/javascript applications and then port to the Windows 8 Metro hooks/apis. As such, is there a tool that can automatically Metrofit your HTML5 web app? (Note: the web application would likely need to be self-contained browser app without a traditional backend for it to work seamlessly).
Official “Migrating a Web app” Instructions:
http://msdn.microsoft.com/en-us/library/windows/apps/hh465143.aspx
Sample “Windows 8 Metro html5 / javascript” project:
http://www.codeproject.com/Articles/338916/Windows-8-JavaScript-Metro-Application-Getting-Sta
No, there is no such tool. You will have to do all the grunt work yourself.