For those looking to enforce Field Level Security for a CRM deployment, Microsoft has come up with a whitepaper that will assist you in coming up with a workable solution. Understanding the architecture, Access Channel's & Access Method's available to users coupled with the introduction of plug-ins may help you in deploying FLS.

 

There is also a couple of handy code snippets!

 

To download the whitepaper click on the link: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=471f8670-47b3-4525-b25d-c11a6774615c