Silverlight or WPF/E
Silverlight is sexy. I have to admit, although I am somewhat afraid of it. Microsoft promises interoperability, and I've seen it work just fine in Firefox, I haven't seen it working on the Mac yet, but I'd bet that it works just fine, although I'm sure you have to have Windows Media Player installed.
Here's why its dangerous, or what I don't like about it. Its another way for Microsoft to lock you in. Microsoft is clearly moving to becoming a platform company, an enabler for others to build products on the Windows stack. Visual Studio is good, almost too good. Of course, the Silverlight SDK will be free, which already puts flash at a disadvantage. Then on top of that the Visual Web Developer Express IDE is free, and its good. What happens is that without truly realizing it, you get used to working with IIS and the Microsoft software stack. You'll be crowing about how easy it is to build dynamic web applications with Microsoft's products. Maybe you'll even get them to spring for Visual Studio, the full version. So, you say, what's wrong with that?
It give Microsoft leverage to destroy its competitors. If Microsoft can get everyone to abandon HTML for Silverlight, then after a while, it doesn't have to make it quite so cross-browser compatible. It would be subtile too, perhaps they would lag versions a little for Firefox and other web browsers. Maybe a little message prompting the user to upgrade to Windows Vista for the best experience on the web. So the result is they will get all of the users to upgrade to Vista and run it on IE 7, especially since you don't have to install anything to make it work. That is the problem with having a vendor that has a vested interest in selling its other products defining a standard. Of course the standard will be optimized for their products. With Microsoft's history, that would probably sound the death knell of anything but Windows, Office, Internet Explorer.
So what's wrong with Microsoft having a superior product and selling it? Nothing, its what I'd expect them to do. They are a company, they have to gain more profits. That's what its made to do. But as an independent web developer, I'd hope that everyone would think long and hard about what their technology choices mean for the future. Do you want Microsoft or open standards? Silverlight is the result of good competition from Macromedia, now Adobe, and the tools and techniques collectively known as AJAX. If you kill that competition, you'll get IE 6 for 5 years again.