http://msdn.microsoft.com/en-us/library/windows/apps/br211386.aspx
"If you know web development technologies, you can develop a Metro style app using HTML5, Cascading Style Sheets, Level 3 (CSS3), and JavaScript."
Microsoft has a page where they discuss Metro-style JavaScript/HTML/CSS apps.
http://msdn.microsoft.com/en-us/library/windows/apps/br229576.aspx
The technology they talk about seems entirely standard, that is, there nothing Windows-specific on that page. So it should be fairly easy to keep things portable when necessary.
You can also use the XAML based interface, which is Windows specific.
http://msdn.microsoft.com/en-us/library/windows/apps/br211377.aspx
I'll have to play around with this stuff at some point, but yeah it is quite exciting to be able to use the same technology I use to write web apps to write Windows 8 apps. 
