Betanews has an article about Microsoft’s “Windows Presentation Foundation Everywhere,” or WPF/E initiative. It’s essentially XAML for other platforms besides Windows Vista. At the PDC conference, Microsoft showed examples of WPF/E on a PDA and smart phone. Support for Mac is planned as well.
This is an important shift for Microsoft. Rather than force everyone to their platform, they’re opening things up a bit. Of course, the native Avalon stuff will have a richer API.