@LCARSNxG: Since there's been nothing earth shatteringly official from anybody other than an offhand comment by Bob Muglia (not in charge of Silverlight?), we can only extrapolate so much. My thinking on this is that they are taking a wait and see approach for HTML5 being a viable cross platform environment. They are putting a lot of effort into it to make sure that they aren't going to get left behind (IE 6, 7, and 8).
The answer that is relevant to you is "it depends". HTML5 is far from being ubiquitous, and probably will never be adequately consistent across all platforms. If it absolutely has to work across all browsers, on the desktop and on mobile devices, then HTML5 might work. However, that's not going to happen for several years. You also have to ask yourself if what you are doing is easier to do in HTML5, is maintainable, and extensible enought to invest in.
Bob Muglia said it himself, Silverlight is here to stay. It is the platform for WP7, and it has a place in certain kinds of applications. Whether your application is one of those, only you can say. I personally have no qualms about keeping my stuff on Silverlight and starting new Silverlight projects. HTML is a mess, and 5 doesn't make it any better.
I've always had a fairly low threshold of pain for the special kind of torment that HTML brings. HTML is a thing from the other side of reality that is all tentacles and eyes and snapping slavering jaws belching gobbets of putrid bile. From the depths of it's writhing black form comes a hideous wail that carries with it the full force of its anguish and hate for all that is alive. Within it's wretched and blackened mind it carries a malevolent purpose. Not content to crush, destroy and burn all in it's path, it seeks to rend it's victims and extract the very soul leaving behind a desiccated and worthless husk, filled only with an eternity of sorrow.
At least that's what happens every time I get involved in an HTML based project. *shrug*