This is part II of the series. Part I can be found here.
Ok, so you want to target multiple mobile OSes and not restrict yourself to Windows 8. Well if you are a web developer you already have a head start. HTML5 is the hot new kid on the block and almost all mobile browsers support part of the HTML5 spec (well HTML5 is not yet a standard, so what is supported is completely arbitrary, but seems like the different mobile browsers support enough in common to be usable).
Also there are a number of other JS libraries that are used quite extensively now, not specifically only for mobile, so it would make sense for you to extend your skillset to stuff like AngularJS, node.js or DOJO.
So using HTML5/CSS3/JS and ASP.NET skills you can build websites that are optimized for the mobile. Also ASP.NET MVC4 has device features that will allow you to have views specific to different devices and thus render a special view based on the device used to browse.
The only gotcho in this approach is that your web site remains a website and you would access it via the browser on the mobile device. As users get more accustomed to devices, the requirement/expectation of using an app rather than launching a browser is increasing.
Also some device specific features like camera, push notifications etc., are not usable from a pure web application.