It seems that the proprietary hooks in .Net are becoming more and more clear. This Netcraft interview with Miguel de Icaza ends with the comment:

Longhorn has kind of a scary technology called Avalon, which when compounded with another technology called XAML, it’s fairly dangerous. And the reason is that they’ve made it so it’s basically an HTML replacement. The advantage is it’s probably as easy as writing HTML, so that means that anybody can produce this content with a text editor. This is the great proprietary catch that ties .Net solely to the Windows platform. No language is useful without a solid set of libraries to work with and the best libraries for .Net (XAML and Avalon) will be Windows only and managed solely by Microsoft, not a standards organization. It’s not like people haven’t warned about this before though…. Some people still think they can get a good deal out of the devil.