Hello
We have been using Adobe Flex for a major project but it's just too slow.
We're considering switching to Silverlight.
The problem seems to be that Silverlight is not as widely supported by browsers as Flex/Flash is.
For example I tried to access the Silverlight showcase using Safari for Windows but it didn't work.
A web RIA platform surely has to have browser ubiquity as its foundation?
Browser Plugins are essentially native applications, so you essentially need a Plugin for each architecture.
For example, there is no 64-Bit Flash Plugin for Windows, so I always have to use 32-Bit IE for that. I also believe that there is no Linux-PPC support, but it's been a while since I've looked at that. And that new JavaFX stuff from Sun also does not run on many platforms and browsers.
For a list of supported Platforms, Wikipedia has a neat table:
IMHO Flash is the only thing that comes near perfection. Microsoft is just supporting a few major browsers.