The W3C has posted its working draft for HTML 5.0. Interesting statement here:
1.1.3. Relationship to XUL, Flash, Silverlight, and other proprietary UI languages
This section is non-normative.
This specification is independent of the various proprietary UI languages that various vendors provide. As an open, vender-neutral language, HTML provides for a solution to the same problems without the risk of vendor lock-in.
Food for thought as you embark on your Flash or Silverlight project. I would have thought XUL is less proprietary, but still.
The contentious part is this:
HTML provides for a solution to the same problems
I doubt that HTML 5.0 can quite match everything that you can do in Flash or Silverlight, depending of course what is meant by “a solution to the same problems”. The other issue is that Flash is here now, Silverlight is just about here now, and HTML 5.0 will be a while yet.