I think I found out when this behavior happens, namely when the IsBusy property is set (to "True") BEFORE the control is loaded in the visual tree. While this might seem a little far-fetched :), in my case it isn't so, here's the order of events:
1. User is on a page.
2. User clicks a button to navigate to another page
3. (some) Framework resolves the navigation request somehow, let's say it decides it needs to load Page2.xaml and hook up its Page2ViewModel.
4. Framework creates view+view-model:
var page = new Page2();
var vm = new Page2ViewModel();
page.DataContext = vm;
5. So the framework supports view models that are ISomeInterface aware and calls their Initialize method. Note that at this point the view still isn't in the visual tree.
6. Initialize() method of the view-model sets some IsLoading property to True, to which the IsBusy property of RadBusyIndicator is bound. Then the vm starts loading some data on another thread.
7. Finally the Page2 view is loaded in the visual tree.
8. The data loading in the view-model ends and the vm sets the IsLoading property to false.
The problem appears between steps 6->8. As far as I can tell what happens is that RadBusyIndicator's OnApplyTemplate method is called, which calls the ChangeVisualState() method. But at this point the IsBusy property is true and the ChangeVisualState() method goes to the Busy state which disables the "background". Theoretically this should be no problem, because as soon as the data is loaded the IsBusy property is set to false, which causes the IsBusyIndicationVisible property to be set to false. However, it looks like when this is done the property changed callback for the IsBusyIndicationVisible property is not called, and so the ChangeVisualState back to Idle is not called either, resulting in the disabled "background". Why this doesn't work I don't know, but maybe you can tell me :)
I hope I'm making some sense here, I have a small repro app which you can get from here: http://cid-5a652c421a7d71ae.office.live.com/browse.aspx/telerik?Bsrc=GetSharingLink&Bpub=SDX.Docs