@BitFlipper: I disagree on that. For small/medium sized applications, like the ones shown in the windows 8 demo video it's not. I agree on the fact that currently it also feels unreal to build a HTML5/js app once and be able to run it on 'every' device. (See for example how crappy iphone apps run on an ipad, the zoom and sizing issues are just crappy) But it feels more realistic than getting silverlight on all of those devices. And I assume with Mango (or the next version) most of these small html5 apps will be able to run on WP, because it includes IE9.
While I agree that Silverlight and it's tooling is miles ahead of html5 developent and I would rather build Silverlight apps than html5/js apps. It still allows Microsoft to tap into a larger developer audience, that is at the moment developing html5/js tablet or phone apps. Which will allow them to create a stronger marketplace of apps.
To me they didn't tell the world, we've dropped Silverlight or WPF development, they've showed one small thing and everyone is screaming. I assume building Silverlight or WPF tiles will just be possible. Than this is a smart move to get a stronger platform, because applications are important for that, without them you have an empty box. I own a WP7 and love it, but needs more apps (Don't know if that is because there isn't a marketplace in my country).