Thank you for writing.
As my colleague Peter has mentioned in the other thread
(Richard Slade has given a link to in his post above) the RadListBox is obsolete since Q2 2010. I would suggest that you upgrade your controls to the latest version and swap your RadListBox with a RadListControl. Test your case and come back should any issues occur.
I hope this will be useful. Should you have further question, I would be glad to help.
the Telerik team