Microsoft has released a new Community Technical Preview of Avalon. You can check out the SDK documentation here. While I look forward to the day when Windows apps are written to Avalon, I still think Macromedia Flash has the brightest future.
It's simple enough. Flash is cross-platform, it's everywhere and you can make some fantastic UI's. Check out the video's of the Macromedia MAX 2004 Conference. They claim that it only takes 12 months for a new version of Flash to be installed by 80% of their massive installed base. Their tools are getting better and broadband is making Flash more useable. Flash is the most popular video player. Flash has a huge and well supported developer community. With web service support, Flash is making inroads as a client to Enterprise Business Apps. However, the current focus of Macromedia is supporting Flash on mobile devices which they claim is a $100 billion dollar market over the next four years.
So if you want both richness and breadth today, it's Flash. And if you want it tomorrow, it's probably still going to be Flash. I've signed up to take a course on Flash next month.
I expect Microsoft to lead the way to a higher abstraction in UI development along with work by others of UI XML schema's. That's great news since it's the main way to improve productivity. Maybe a UI XML that targeted Flash would give us the best of both!