I get asked that question a lot. And frankly, it's a valid question that requires a thoughtful answer. At face value, it looks like Microsoft isn't practising what it preaches - if Microsoft doesn't even switch out Flash, why should you as a RIA solutions provider? I can assure you that this is something that we at Microsoft are very aware of.
If you go to some Microsoft web properties (among the most notable ones being microsoft.com and silverlight.net), you'll see that we are using Silverlight as our RIA platform. There are other Microsoft sites, however, that still use Flash (like xbox.com and zune.net). Does this mean we are hedging our bets on Silverlight because we're not sure the platform will fly? The answer is absolutely and unequivocally no! We are completely dedicated to Silverlight and its adoption.
The reason why Microsoft hasn't gone completely Flash-free is one of making sure the transition is smooth - like almost every company out there, there are a lot of technology assets that exist within it and switching out one technology for another isn't necessarily an easy thing to do. Microsoft is very cognisant of the fact that ensuring the experience users have on a web property it owns is seamless and enjoyable. In order to provide that kind of experience, there are more things in play than just technology - there's the business side of it (making sure "the lights stay on through and after the transition") , the marketing side of it (are the right messages in place with the new technology?) as well as the skills side of it (it's a new platform and our partners who we contract to build the RIAs have to learn it, too!). Obviously moving to a new platform is something that requires careful thought.
Denny Boynton, a Microsoft Architect Advisor out of the US posts a very interesting and insightful article about this topic which I invite you to take a look at.