It was interesting to read this CNet.com article around Flash, HTML and AJAX.
It talked about the various ways in which the Adobe Flash is struggling to compete with AJAX in the RIA space. In fact, some of our early research showed that AJAX was associated with RIA, and not Flash, despite Adobe's best efforts to claim the ownership behind the acronym.
Example, Ryan Stewart @ Adobe was quoted:
From the .Content onwards, you now have full access to the Silverlight solution as if you were accessing the content from within C#, IronJava, IronRuby etc.
This again, illustrates how committed we are to ensuring Silverlight is an open cross-platform, cross-browser and cross-device solution for your next bet. As opposed to a closed binary format which has restrictions implied such as the External Interface API.
Further, if .NET isn't your cup of tea, that's fine you can add your own dynamic language support to Silverlight, via the DLR such as the OpenSource Project that is currently working on enabling Client-Side PHP to work ontop of the DLR.