Seems like cool stuff. I suppose the only downside is lack of support on Linux unlike both Flash and Ajax.

Quoted: Microsoft have also demonstrated today that their vision is for all browsers and all web users, not just users of Internet Explorer, as a common theme during the keynote presentations was inter-operability with both Firefox and Safari, and working with the Mac OSX platform.
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Today, only 14 days from the original announcement, Microsoft has officially announced that Silverlight will also contain a compact CLR, allowing developers to build desktop like applications on the web in a number of supported programming languages.
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The most remarkable part of the CLR are its speed and its size. First of all, the full Silverlight download with CLR and everything else will weigh in at around 4MB - which with current broadband penetration is effortless. Second of all the CLR is fast, very very fast. In a demonstration today showing a game of chess routines written in .NET competed against native Javascript routines and the result was a speed difference of orders of magnitude. Developers can simple take their existing Javascript and copy it into Silverlight and have it perform multiple times faster than it does in the native browser environment.