private static bool? _isInDesignMode;

/// <summary>
/// Gets a value indicating whether the control is in design mode (running in Blend
/// or Visual Studio).
/// </summary>
public static bool IsInDesignModeStatic
{
    get
    {
        if (!_isInDesignMode.HasValue)
        {
#if SILVERLIGHT
            _isInDesignMode = DesignerProperties.IsInDesignTool;
#else
            var prop = DesignerProperties.IsInDesignModeProperty;
            _isInDesignMode
                = (bool)DependencyPropertyDescriptor
                .FromProperty(prop, typeof(FrameworkElement))
                .Metadata.DefaultValue;
#endif
        }

        return _isInDesignMode.Value;
    }
}