You can use the CellEditiorInitialized to set the desired data source to your combo box editor. Notice that the GridViewComboBoxColumn should have a data source which contains all records that should be displayed in it. You can use the cascading combo box knowledge base article to see how you can achieve a similar scenario.
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download
; also available is the Q2'11 Roadmap
for Telerik Windows Forms controls.