Its seems like everyone reacting to HTML5/JS development in Windows 8. I think this is just 1% of the whole story. Its definitely going to support number of technologies. I'm sure you can build Windows 8 apps in many different technology, eg: Silverlight, XNA, WPF, HTML5, .NET, etc. Since HTML5 is the newly added one, they came out with it first, so worry less.
@ryanhayes: Actually, that might be half true. We can expect Intel-based Windows 8 to run anything that run on Windows 7. However, we don't know much about ARM-based Windows 8 yet. IMHO, it is impossible to have ARM-based Windows8 run existing Windows 7 applications (natively). All new applications have to be re-created for ARM-based Windows 8.
Hopefully, .NET, Silverlight, and WPF applications can be write-once and run on both platforms.
Windows 8 is still Windows, so it should run Silverilght and XNA applications.
The questions are what need to be done to create touch-mode interface application. Silverlight still does not support Touch very well comparing to WPF and native Windows API. Hopefully, this will be changed. We should know more in September.