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With the Silverlight 5 due to be released in a couple of weeks, we can expect to see it around for a year or two more. I think that Microsoft is obligated to support it for at least a year after it is launched.
However, I would bet that Silverlight mobile (or whatever it is that Microsoft calls the version for Windows Phone) follows Flash into the dustbin. Microsoft has nowhere near the level of adoption for Silverlight that Flash did for mobile platforms, and I think it has one of the same significant issues that hurt Flash - it is not as energy efficient as HTML5 on mobile platforms. I don't see any reason why HTML5 won't become the de facto standard in the near future.