As demand for richer user experience grows are we going to see more Ajax-centric companies align with a RIA framework that enables more functionality? According to Tibco they chose Silverlight because they could leverage existing skills in C# and that Microsoft and Tibco have a lot of joint customers.
We're seeing a lot of RIA adoption across a number of verticals including the enterprise, where I think Tibco has a lot of strength. Dojo has been pushing the envelope for animations in it's Framework and added some support for Silverlight recently.
I'm not sure how significant the Tibco announcement is by itself, but it may be the start of a trend. Getting the RIA vendors closer to a number of Ajax frameworks is always a good thing because I think it gets the communities talking and sharing ideas which makes all RIA platforms, Ajaxy and not, better.