I have read a range of articles on advantages/disadvantages of Microsoft Silverlight framework in comparisson to Flash.
Fact that there were two version of Silverli
I have integrated both Flash and Silverlight into my ASP.Net applications. I have to say that working with Silverlight was by far the easier way to go. Communication with the server is easier, initial setup was easier (along with a ASP.Net application or a simple HTML page). Integration into the ASP.Net application was the easiest part (as I'm sure you imagined it would be). If you're a .Net developer, then this is the way to go.
Also, I wouldn't worry about either of these technologies going away anytime soon. HTML5 is not going to take over. It'll take some of the market share, not all.
EDIT
Here are some links to other SO posts on the subject: