I love Dependency Properties. Especially attached properties. There is just something extremely cool about being able to think up some property and then setting that in a datatemplate. For instance, creating a custom panel with your own attached properties, and reading these properties during the measure and arrange bits of the layout.
But when you do a get on the property, you will find your left with an unset value!

The backing mechanism for such properties use the object you set them on as a key. When you only have a UIElement, you can not read the value using the UIElement, because you never bound the property to a UIElement, but to your own visual in the datatemplate.
So always remember to use the original object when you are getting your value.

In the case of the UIElement in an itemscontrol, you will find that it is actually a wrapper around your original visual. Get to it by using the infamous visual tree helper:

DependencyObject o = VisualTreeHelper.GetChild(uie, 0);

Then use 'o' in your getvalue!