Recently I had a question about adding security at the column level.  Here is a good article to review.

"This article has shown how to extend SharePoint to include column-level security and still keep all of the data within SharePoint. The strategies identified in this article enable the seamless addition of this functionality into any SharePoint environment." 

 Full Article:

 http://www.infoq.com/articles/Dressel-Gogolowicz-wss-security

MSDN Dowload:

http://code.msdn.microsoft.com/SecureField 

Good luck.