If you're an HTML5 developer, then you're all set.
If only this were true =(
Unfortunately the learning curve of HTML5 Windows 8 apps is too great, I don't think it will find the adoption Microsoft wanted. And we have first hand experience with this, building an internal Windows 8 app for Microsoft, done in HTML5/WinJS....NOT a fun experience. Our next Windows 8 app is going to be built in Xaml/WinRT, so hopefully a much better experience =)