I did wonder for a few days what RIA was and now I know (Rich Interactive/Internet Application). Thanks to MOSSYBLOG
What he is trying to say is that silverlight is not a competition for flash, but that silverlight is the missing bit in the whole Microsoft ecosystem. I think he is right. I also think Microsoft is coming out with the right tools to get the development done and at the right levels. Specifically in that it is possible to get content into silverlight with a text editor only. In the same way you can develop c# applications with a text editor only. ha!. If you really want to. I suspect its not if you WANT to, more like, if you have no choice and cannot afford the IDE. That problem has been removed with the release of the express products.
All in all, silverlight plugs one of the reasons for using a none ms technology.