Thank you for writing.
Our RadTextBox supports only the AutoComplete mode supported by the standard Microsoft TextBox. The feature request that you have is logged in our Public Issue Tracking System. Feel free to add your vote for it here.
Write back if you have additional questions.
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.