I'm using Visual Studio 2010 Express, working on a Silverlight 4 project in C#. This started happening all of a sudden in my project, I get the error that this XAML Namespace is not resolved:

XAML Namespace http://schemas.microsoft.com/winfx/2006/xaml is not resolved

If it helps, here is the section of the XAML file in which the error is being raised:

<ResourceDictionary xmlns:my="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:GetGlue="clr-namespace:GetGluePlugin;assembly=GetGluePlugin"
                xmlns:System="clr-namespace:System;assembly=mscorlib"
                xmlns:utils="clr-namespace:Seesmic.Sdp.Utils;assembly=Seesmic.Sdp.Utils">

What could the problem be?