Thank you for writing.
Currently, we do not offer such functionality for our RadTextBox. However, you can easily achieve this scenario with our controls without much trouble. I have attached a sample project with two password strength indicators. One uses the RadProgressBar and the second one the RadLabel for indication. They both use the same algorithm to calculate the password strength which you should change, so it would comply to your requirements. In addition, you should be able to easily change the visual indication of the password strength.
I hope you will find this useful. If you need further assistance I would be glad to provide it.
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.