Some time ago, I wrote a whitepaper that discusses using Elevated Privileges and Impersonation in SharePoint. The paper and related code are available on the Mindsharp web site in the Premium Content section. (Getting the paper will cost you some demographic information and a follow-up phone call.)
The paper is the basis for a talk at the SharePoint Best Practices conference. We're doing another conference in February in San Diego, CA. Bill English has the inside story.