Quick fix for a problem that was haunting me today:
If you upgrade an ASP.NET 2.0 app to 3.5 and have .xaml files in your project that are part of a Silverlight 1.0 or 1.1 project, the conversion wizard converters them to Build-type: “Page” and adds a custom build for building the WPF files. If this happens you’ll get a cryptic error:
“error MC6000: Project file must include the .NET Framework assembly ‘WindowsBase, PresentationCore, PresentationFramework’ in the reference list.”
Go through your xaml files and mark them as “Content” and remove any custom tools and it will fix it.