Please, accept my apologies for the curt reply. I will try to explain a bit more - there is a bug in the Silverlight 2 Popup control, that mixes the namescopes of the loaded inside the Popup UserControls. Usually, each UserControl instance has a separate namescope, e.g. all Name attributes inside it are unique and do not collide with the Name attributes of other instances of the same UserControl, or other UserControls. However, there are some situations where the Silverlight 2 Popup "mixes" the namescopes of the UserControls that are placed inside it. This is the same problem you are experiencing. To avoid it, you should not set Name on the combo box items. If you need to keep some data for each combo box item in order to distinguish them, you could use the Tag property instead. Other option would be to databind the combo box.
This bug is fixed in Silverlight 3.
All the best,
the Telerik team