Yes, due to the lazy manner in which Silverlight and WPF controls are loaded, it is not trivial to select an item by its value (and not its path).
Almost certainly this will not be a built-in feature of the TreeView and it has to be done in the underlying ViewModel.
The problem is that the TreeView has no idea of the existence or value of items in the branches that have not been expanded yet. With the Q3 release the TreeView will be virtualized, which means that only the visible 20-30 item containers will be present - this will improve the performance but it will mean that increasingly containers will not be available. Then, if there are no containers, there is no reasonable way to get the subitems of the root-level items.
Thank you for the reminder, I will prepare an example with the different cases of getting an item.
I will reply to your other thread as well with more concrete ideas for your scenario.
the Telerik team