It appears that after installing Windows XP SP2, InterDev 6.0 no longer allows right-clicking on the control in design mode to bring up the properties dialog box.
That behavior is quite specific to InterDev 6.0 and affects all ActiveX controls, including Microsoft's.
All other environments continue to allow right-clicking on the control to access the control's properties. .NET, VB, VC++, etc. all work as expected.
InterDev 6.0 still accepts Alt+Return to access the property pages and of course through the "Custom" entry of the IDE's Properties window and the property dialog button. Since right-click support is provided by the IDE (not our control), we have to assume some change to InterDev has been made by Microsoft or at least some of the new security settings have an adverse effect on InterDev 6.0.