I think I'm breaking your code. Don't have more time to play with it right now, but here's what I have:

XAML:

<Window x:Class="WpfPropertyInheritance.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:loc="clr-namespace:WpfPropertyInheritance"
    Title="Window1" Height="300" Width="300">
    <StackPanel>
    <Button Height="23" Name="button1" Width="75" Click="button1_Click">Button</Button>
    <Button Height="23" Name="button2" Width="75" Click="button2_Click">Button</Button>
    <loc:ShapeCanvas3 x:Name="scTest" Height="172">
      <StackPanel Canvas.Left="46" Canvas.Top="38" Height="100" Name="spnlTest" Width="200" >
        <Rectangle Height="100" Name="rectangle1" Stroke="Black" Width="200" Fill="Azure" />
      </StackPanel>
    </loc:ShapeCanvas3>
  </StackPanel>
</Window>

C#:

using System.Windows;
using System.Windows.Controls;
using System.Diagnostics;

namespace WpfPropertyInheritance
  {
  /// <summary>
  /// Interaction logic for Window1.xaml
  /// </summary>
  public partial class Window1 : Window
    {
    public Window1()
      {
      InitializeComponent();
      }

    private void button1_Click(object sender, RoutedEventArgs e)
      {
      Debug.WriteLine("A1 Starting Value: " + scTest.ContentIsLocked.ToString());
      scTest.ContentIsLocked = true;
      Debug.WriteLine("A2 Set Canvas True: " + scTest.ContentIsLocked.ToString());
      scTest.ContentIsLocked = true;
      Debug.WriteLine("A3 Set Canvas True Again: " + scTest.ContentIsLocked.ToString());
      scTest.ContentIsLocked = false;
      Debug.WriteLine("A4 Set Canvas False: " + scTest.ContentIsLocked.ToString());
      }

    private void button2_Click(object sender, RoutedEventArgs e)
      {
      Debug.WriteLine("B2 Starting Values: " + scTest.ContentIsLocked.ToString()+ ":" + rectangle1.GetValue(ShapeCanvas3.ContentIsLockedProperty).ToString());
      scTest.ContentIsLocked = true;
      Debug.WriteLine("B2 Set Canvas True: " + scTest.ContentIsLocked.ToString() + ":" + rectangle1.GetValue(ShapeCanvas3.ContentIsLockedProperty).ToString());
      rectangle1.SetValue(ShapeCanvas3.ContentIsLockedProperty, false);
      Debug.WriteLine("B2 Set Rect False: " + scTest.ContentIsLocked.ToString() + ":" + rectangle1.GetValue(ShapeCanvas3.ContentIsLockedProperty).ToString());
      }
    }

  public class ShapeCanvas3 : Canvas
    {
    public static DependencyProperty ContentIsLockedProperty;

    static ShapeCanvas3()
      {
      FrameworkPropertyMetadata p = new FrameworkPropertyMetadata(
        new PropertyChangedCallback(OnContentIsLockedChanged), 
        new CoerceValueCallback(CoerceIsLocked)
        );
      p.Inherits = true;
      ContentIsLockedProperty = DependencyProperty.RegisterAttached("ContentIsLocked", typeof(bool), typeof(ShapeCanvas3), p);
      }

    public bool ContentIsLocked
      {
      set {
          Debug.WriteLine("SetVal: " + value.ToString());
          if (value)
            SetValue(ContentIsLockedProperty, value);
          else
            this.ClearValue(ContentIsLockedProperty);
          }
      get { return (bool)GetValue(ContentIsLockedProperty); }
      }

    private static void OnContentIsLockedChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
      {
      }

    private static object CoerceIsLocked(DependencyObject d, object value)
      {
      if (d.ReadLocalValue(ContentIsLockedProperty) == DependencyProperty.UnsetValue)
        {
        Debug.WriteLine("Coerce 1: "+d.ToString()+":" + value.ToString());
        return value;
        }
      bool b = (bool)value;
      if (b)
        {
        Debug.WriteLine("Coerce 2: " + b.ToString());
        return b;
        }
      else
        {
        Debug.WriteLine("Coerce 3: UnsetValue");
        return DependencyProperty.UnsetValue;
        }
      }
    
    }  
  }

Here is the debug output on the second button:

B2 Starting Values: False:False
SetVal: True  
Coerce 1: WpfPropertyInheritance.ShapeCanvas3:True
Coerce 1: System.Windows.Controls.StackPanel:True
Coerce 1: System.Windows.Shapes.Rectangle:True
B2 Set Canvas True: True:True
Coerce 1: System.Windows.Shapes.Rectangle:False
B2 Set Rect False: True:False

B2 Set Rect False: True:False = Canvas is locked, but child was able to be unlocked.

Am I missing something?