Column Level Security in SharePoint

Windows SharePoint Services 3.0 and Microsoft Office SharePoint Server 2007 support a rich security model that allows administrators to control access to sites and content by assigning permissions to users and groups for a specific securable object (such as site, list, library, folder and even an individual document or item).

Currently, SharePoint does not provide out of the box support for securing columns or views.
Here there is good article for securing the columns in SharePoint.
(http://www.infoq.com/articles/Dressel-Gogolowicz-wss-security)

Its working because I used in SPList & Discussion Board.
But there is limitation for lookup field (255 characters) so you can secure item's fields value up to 255 char.