I see a lot of talk how HTML5 video tag will kill Flash. But while video is the most widely used part of Flash/SL, it's only a small part of their technical abilities. For instance you can write a game using full 3D graphics and socket connections in Flex, and serious business applications, etc.
Is the thinking that Javascript will kill those parts of Flash/Flex/SL? Because while that seems feasible now for even quite rich web-apps, what about any kind of high-performance app like real-time graphics?
The discussions I've been part of on this subject, pretty much always seem to come to the conclusion that, while the HTML 5 standard will be a a great thing, once it's in place. It will by definition be a standard, standards take a long time to change, therefore innovation will be driven largely by plugins like Flash and SL.
And yes you can do a lot of what Flash and SL can do with Javascript, however Flash/Flex has the strong point of being used for desktop installed Air applications, and Silverlight has the similar thing with their "Out Of Browser" functionality. These are things where they clearly have the upper hand over Javascript.
At the end of the day it's not a matter of who will kill who, but rather which tool is the right one for the job and what skills does you / your development team have.
That's my 2c anyway...