Within WSS, items can be unsecured (available to the anonymous user), secured to specific users or groups, or generically secured to All Authenticated Users. If an item (or list) is accessible to All Authenticated Users, in most intranet cases it is a near equivalent to unsecured.

To impersonate a user, you must create a new SPSite object on behalf of the user and access items from within it’s security context. You cannot use the SPContext.Current scoped references of the SPSite or SPWeb. This is true whether you are impersonating an arbitrary user or are using SPSecurity.RunWithElevatedPRiveleges. To open an impersonated site, you would use the SPSite contructor that takes a site ID (GUID) and user token. Note that the user token is an opaque object that you get from WSS, and is implemented distinctly on Windows authenticated servers where the token represents AD group information, and on Forms authenticated servers where it simply stores the user ID.

To get a token to impersonate, use the "AllUsers" property of the SPWeb. This group includes all users that WSS knows about in this site context. To get the All Authenticated Users user, use the following:

// Gets the All Authenticated Users SPUSer
SPUser spUser = SPContext.Current.Web.AllUsers[@"NT AUTHORITY\authenticated users"];

With this SPUser reference, you can now use its token to open an impersonated SPSite. Be sure to dispose of these objects as they were not created as part of the WSS Http pipeline.  

SPUserToken token = spUser.UserTooken;
using (SPSite impSite = new SPSite(SPContext.Current.Site.ID, token))
{/* …use the impersonated site context. */}

 *Note that I have not verified user impersonation with MOSS, as I’ve been focusing on cross-environment (WSS and MOSS) platform APIs lately. I’ve been getting a few comments that MOSS impersonation seems to work differently, so I will have to check into that! If you have code examples of impersonation using the MOSS framework additions please leave a comment and I’ll add a link.  

Read more: Page 372 of Inside Windows SharePoint Services