This feature is currently not supported by the RadTreeView component. As Richard said, we are currently refactoring RadTreeView planning to release the updated component in Q1 2011. We will include this functionality and the possibility to customize the different states of the checkbox when the control is in this mode.
Thank for your comments and feedback.
All the best,
the Telerik team
Q3’10 SP1 of RadControls for WinForms is available for download
; also available is the Q1'11 Roadmap
for Telerik Windows Forms controls.