Thank you for bringing our attention to this issue.
I tested the scenario and found that this is actually an issue in some of our themes. It appears in Office2010Black
themes. I have logged this issue in our issue tracking system. You can track its progress by following this link
I have updated your telerik points accordingly. If you have further questions I would be glad to help.
All the best,
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.