If you have downloaded ASP.NET Futures (July 2007) recently (I.e. version 1.3.61025), you might face following issue and this post described a workaround to mitigate the issue.
Issue:

When you use SilverlightStreamingMedia control, you might hit compilation error. For example, if you are defining chapters collection for your video content, you would hit compilation error.

Here's the declaration of SilverslightStreamingMedia control on default.aspx after drag and drop from toolbox and defining a chapter "Chapter1" for chapters collections in property grid.   

slsmedia declaration with chapter   

When you compile this, following error are reported.   

slsmedia version error list    
Workaround:
  • First, register Microsoft.Web.Preview assembly on the web page hosting SilverlightStreamingMedia control, by adding following line at the top of the web page:

sls media tagprefix

  • Remove the chapter definitions and re-add the chapter definitions. The above declaration would, now, look like following and would compile without errors in Visual Studio 2008 RTM release.

slsmedia declaration with chapter -correct

Note: This issue is found only if you have installed ASP.NET Futures (July 2007) version 1.3.61025 (or later) 

We are working on updating the Nov 2007 CTP release to fix this issue. If you have bugs or have feedback for the Windows Live Tools, you can provide your feedback at Windows Live Dev Connect Feedback site.