Is Silverlight a Flash killer?  Not really.   Sure, flash is gonna die, but that's not what Silverlight has its sites set on. 

Silverlight IS AN HTML killer.  It's a tool for total Web revolution.  Silverlight, or a technology that provides what it provides, will bring us the WebOS. 

Why?  Well that will require several posts, but first let me address why calling it a Flash killer (or Java killer if you like), is doing Silverlight a disservice. 

Flash

Flash was an almost complete an utter failure.  That's a strong accusation to levy against the most popular browser plug-in in the world, so I'd better explain myself.  Why didn't Flash become THE web platform?  The graphics are better, the interaction is fluid, the support for web services is there.  So why do you only see it on banner ads, widgets (YouTube videos, Photo slide shows etc), and movie web sites?  Because Flash was meant for designers.  I've no doubt it does a best in class job in this arena.  However, designers don't program, and they don't handle data.  Lets face it, geeks are important to the equation if you want to create a site people come back to.  Adobe/Macromedia never invited the geeks.  As a result, flash has been relegated to a subservient position to, of all miserable, out-dated technologies, HTML/CSS/JS.  (It's possible that Flex is supposed to be Adobe's Silverlight, but I know nothing about it.  I don't think I'm unusual in that ignorance.  Most developers gave up on Flash years ago.  They've had at least a decade to run with this unimpeded, and they let it slip.)

Java

While I'm at it, what about Java?  Wasn't it supposed to be the killer web platform also?  Yeah it was, but it suffered from the exact opposite problem.  Java let developers do very intense, powerful things in a browser.  3D interactive web trees, chess games...  flat gray buttons.  Java on the browser is ugly.  Really REALLY ugly.  Designers weren't invited to that party. 

So the moral of the story is that designers and developers are both critical pieces to the puzzle.  You can't do anything compelling w/out buy-in from both sides.  Enter Silverlight.

Silverlight

Microsoft has created a platform that provides development tools to the developer, designer tools to the designer, and a data format that tries to be as agnostic as possible.  They saw the problem, and designed a solution to solve it. 

So this really just explains why Silverlight will sweep Flash aside as the dominant rich media platform.  It hints at why Silverlight will likely begin overtaking HTML/CSS/JS, but there are 2 other very critical pieces to this story.