We are also encountering "catastrophic failure" in the RadComboBoxes when we try to change the selected value.
I notice that this is happening when we are setting the StyleManager.ApplicationTheme to the the following themes, Office_Blue, Office_Silver, Summer, Vista
StyleManager.ApplicationTheme = Telerik.Windows.Controls.ThemeManager.FromName(TelerikTheme.Office_Blue.ToString());
Office_Black is working fine.
We are using the Q32009 release.