With Q2 2009 we fixed a bug that allowed RadComboBox to have SelectedItem = something that is not in its Items collection. The same behavior can be observed with the standard ComboBox. I would recommend checking whether the object you are setting as value of the SelectedItem property is inside the Items collection of the control.
Regarding the "Find All References" problem - we never experienced such crashes with our demos, sample applications or internal Silverlight projects. Could you please send us a simple project that could be used to reproduce the problem and we will respond with a solution ASAP.
the Telerik team