This problem occurs with the latest 2012 Q1 release.
If I use the Silverlight toolkit extension method GetItemsAndContainers (System.Windows.Controls.Toolkit.dll) in the DropDownOpened event, the first time this event fires I get 0 items back from this call. It occurs if you bind an IEnumerable to ItemsSource or even explicitly declare strings inside RadComboBoxItem tags.
sender, EventArgs e)
RadComboBox rcb = (RadComboBox) sender;
, DependencyObject>> itemsContainers = rcb.GetItemsAndContainers();
RunOpenCount.Text = itemsContainers.Count().ToString();
This worked in 2011 Q3 and quite a few prior releases too.
Here is the solution showing the problem: http://dl.dropbox.com/u/15525269/ComboBoxIssue.zip