If you are feeling the heat from other platforms, what do you do?
You put your rich experience into many browsers, so that HTML compatibility is no longer an issue, and neither is a different operating system.
Silverlight 1.0 beta has a subset of Windows Presentation Foundation. 1.1 alpha (a 4.25MB download) has a subset of the .NET framework (a 4.24MB download for the alpha).
This brings the rich smart (and the whole developer infrastructure behind it – once the tools come out) right across the board in terms of platforms.