We confirm the issues regarding VS 2005 design time behavior with custom controls, however they are not related to Telerik per se. You will experience similar issues with other 3rd party custom controls.
We are still researching how Microsoft have managed to overcome this problem with their controls and we will fix all issues that arise from our controls in design-time, starting with RadToolStrip
Thank you very much for your feedback. Your points have been updated!
the Telerik team