I have the following (very simple) ItemsControl:
I used the code that Kent and Timothy supplied, but this additional line was also needed for me.
The whole code snippet goes:
var container = _itemsControl.ItemContainerGenerator.ContainerFromItem(dahCurrentItem) as FrameworkElement;
container.ApplyTemplate()
var checkBox = _itemsControl.ItemTemplate.FindName("MyCheckBox", container) as CheckBox;
Cheers!