I've been an avid user of telerik's "RadControls for ASP.NET" for quite some time now. RadControls are essentially a control framework built on top of ASP.NET technology which allows the average ASP.NET punter to produce Rich AJAX driven solutions with skinning capabilities. They have things like "RadWindows", "RadComboBox" and so on, basically a great add-on to ASP.NET AJAX.
Well, they've now seen the light so to speak - they are also producing a new iteration of their framework only this time they are leveraging the power of ASP.NET AJAX and Silverlight. It's name given is called "Prometheus" which essentially is borrowed from Greek mythology (ie: brother of Atlas heh).
I've not yet gotten a hold of the beta (*hint*) but what I liked about this approach is the fact they are essentially combining both AJAX and Silverlight under the one roof.
I foresee the first few cycles of Silverlight development to be hybrid approach, in that either using Adobe Flash or AJAX. Why I foresee this? well it's new for a start and with all new things there is this prelude to greatness if you will. Where crazy stuff gets built and we all sit back, think about it and then evolve it further and further.
Secondly, most folks whom I interact with that are using Silverlight today have both Flash or AJAX skills built in. Whilst Silverlight is offering more and more as it iterates, these folks typically like to keep their existing code snippets or ideas intact as they progress forward down the learning Silverlight path.
It's not a bad thing, it's actually quite exciting as I think the Rich part of RIA will shift slightly and I hope that things will change in the future around how RIA is defined today.
Control Frameworks like the ones Telerik produce are a great start to what I think will be a productive future.